feat: add llms.txt and LLM-friendly markdown docs generation

[takaebato]

Summary

Add llms.txt and LLM-friendly markdown documentation generation to the ECharts doc build pipeline. Since the current SPA-based docs are difficult for AI agents to access via web fetch, this provides static Markdown alternatives.

• Add build/build-llms.js that mechanically converts documents/*-parts/*.json to llms-documents/*-parts/*.md by converting HTML descriptions to Markdown via turndown, with type/default info extracted from full schema JSONs (option.json, api.json, etc.)
• Integrate into the existing build pipeline via build/build-doc.js (runs after main doc build)
• Generate llms.txt index file per language (en/zh) listing all available documentation
• Add turndown and turndown-plugin-gfm as dependencies

Output structure

  en/
  ├── llms.txt
  └── llms-documents/
      ├── option-parts/     (55 files)
      ├── option-gl-parts/  (18 files)
      ├── api-parts/        (5 files)
      └── tutorial-parts/   (1 file)
  zh/ (same structure)

Output examples

en/llms.txt

# Apache ECharts Documentation

> Apache ECharts is a free, powerful charting and visualization library offering easy ways to add intuitive, interactive, and highly customizable charts to your commercial products.

## API

- [api.action](llms-documents/api-parts/api.action.md)
- [api.echarts](llms-documents/api-parts/api.echarts.md)
- [api.echartsInstance](llms-documents/api-parts/api.echartsInstance.md)
- [api.events](llms-documents/api-parts/api.events.md)
- [api](llms-documents/api-parts/api.md)

## Option GL

- [option-gl.geo3D](llms-documents/option-gl-parts/option-gl.geo3D.md)
- [option-gl.globe](llms-documents/option-gl-parts/option-gl.globe.md)
- [option-gl.grid3D](llms-documents/option-gl-parts/option-gl.grid3D.md)
- [option-gl.mapbox3D](llms-documents/option-gl-parts/option-gl.mapbox3D.md)
- [option-gl](llms-documents/option-gl-parts/option-gl.md)
- [option-gl.series-bar3D](llms-documents/option-gl-parts/option-gl.series-bar3D.md)
- [option-gl.series-flowGL](llms-documents/option-gl-parts/option-gl.series-flowGL.md)
- [option-gl.series-graphGL](llms-documents/option-gl-parts/option-gl.series-graphGL.md)
- [option-gl.series-line3D](llms-documents/option-gl-parts/option-gl.series-line3D.md)
- [option-gl.series-lines3D](llms-documents/option-gl-parts/option-gl.series-lines3D.md)
- [option-gl.series-map3D](llms-documents/option-gl-parts/option-gl.series-map3D.md)
- [option-gl.series-polygons3D](llms-documents/option-gl-parts/option-gl.series-polygons3D.md)
- [option-gl.series-scatter3D](llms-documents/option-gl-parts/option-gl.series-scatter3D.md)
- [option-gl.series-scatterGL](llms-documents/option-gl-parts/option-gl.series-scatterGL.md)
- [option-gl.series-surface](llms-documents/option-gl-parts/option-gl.series-surface.md)
- [option-gl.xAxis3D](llms-documents/option-gl-parts/option-gl.xAxis3D.md)
- [option-gl.yAxis3D](llms-documents/option-gl-parts/option-gl.yAxis3D.md)
- [option-gl.zAxis3D](llms-documents/option-gl-parts/option-gl.zAxis3D.md)

## Option

- [option.angleAxis](llms-documents/option-parts/option.angleAxis.md)
- [option.aria](llms-documents/option-parts/option.aria.md)
- [option.axisPointer](llms-documents/option-parts/option.axisPointer.md)
- [option.brush](llms-documents/option-parts/option.brush.md)
- [option.calendar](llms-documents/option-parts/option.calendar.md)
- [option.dataset](llms-documents/option-parts/option.dataset.md)
- [option.dataZoom-inside](llms-documents/option-parts/option.dataZoom-inside.md)
- [option.dataZoom-slider](llms-documents/option-parts/option.dataZoom-slider.md)
- [option.geo](llms-documents/option-parts/option.geo.md)
- [option.graphic](llms-documents/option-parts/option.graphic.md)
- [option.grid](llms-documents/option-parts/option.grid.md)
- [option.legend](llms-documents/option-parts/option.legend.md)
- [option.matrix](llms-documents/option-parts/option.matrix.md)
- [option](llms-documents/option-parts/option.md)
- [option.media](llms-documents/option-parts/option.media.md)
- [option.parallel](llms-documents/option-parts/option.parallel.md)
- [option.parallelAxis](llms-documents/option-parts/option.parallelAxis.md)
- [option.polar](llms-documents/option-parts/option.polar.md)
- [option.radar](llms-documents/option-parts/option.radar.md)
- [option.radiusAxis](llms-documents/option-parts/option.radiusAxis.md)
- [option.series-bar](llms-documents/option-parts/option.series-bar.md)
- [option.series-boxplot](llms-documents/option-parts/option.series-boxplot.md)
- [option.series-candlestick](llms-documents/option-parts/option.series-candlestick.md)
- [option.series-chord](llms-documents/option-parts/option.series-chord.md)
- [option.series-custom](llms-documents/option-parts/option.series-custom.md)
- [option.series-effectScatter](llms-documents/option-parts/option.series-effectScatter.md)
- [option.series-funnel](llms-documents/option-parts/option.series-funnel.md)
- [option.series-gauge](llms-documents/option-parts/option.series-gauge.md)
- [option.series-graph](llms-documents/option-parts/option.series-graph.md)
- [option.series-heatmap](llms-documents/option-parts/option.series-heatmap.md)
- [option.series-line](llms-documents/option-parts/option.series-line.md)
- [option.series-lines](llms-documents/option-parts/option.series-lines.md)
- [option.series-map](llms-documents/option-parts/option.series-map.md)
- [option.series-parallel](llms-documents/option-parts/option.series-parallel.md)
- [option.series-pictorialBar](llms-documents/option-parts/option.series-pictorialBar.md)
- [option.series-pie](llms-documents/option-parts/option.series-pie.md)
- [option.series-radar](llms-documents/option-parts/option.series-radar.md)
- [option.series-sankey](llms-documents/option-parts/option.series-sankey.md)
- [option.series-scatter](llms-documents/option-parts/option.series-scatter.md)
- [option.series-sunburst](llms-documents/option-parts/option.series-sunburst.md)
- [option.series-themeRiver](llms-documents/option-parts/option.series-themeRiver.md)
- [option.series-tree](llms-documents/option-parts/option.series-tree.md)
- [option.series-treemap](llms-documents/option-parts/option.series-treemap.md)
- [option.singleAxis](llms-documents/option-parts/option.singleAxis.md)
- [option.stateAnimation](llms-documents/option-parts/option.stateAnimation.md)
- [option.textStyle](llms-documents/option-parts/option.textStyle.md)
- [option.thumbnail](llms-documents/option-parts/option.thumbnail.md)
- [option.timeline](llms-documents/option-parts/option.timeline.md)
- [option.title](llms-documents/option-parts/option.title.md)
- [option.toolbox](llms-documents/option-parts/option.toolbox.md)
- [option.tooltip](llms-documents/option-parts/option.tooltip.md)
- [option.visualMap-continuous](llms-documents/option-parts/option.visualMap-continuous.md)
- [option.visualMap-piecewise](llms-documents/option-parts/option.visualMap-piecewise.md)
- [option.xAxis](llms-documents/option-parts/option.xAxis.md)
- [option.yAxis](llms-documents/option-parts/option.yAxis.md)

## Tutorial

- [tutorial](llms-documents/tutorial-parts/tutorial.md)

llms-documents/option-parts/option.title.md (excerpt)

# option.title

## id
- **Type**: `string`

Component ID, not specified by default. If specified, it can be used to refer the component in option or API.

## show
- **Type**: `boolean`
- **Default**: `true`

Set this to `false` to prevent the title from showing

## text
- **Type**: `string`
- **Default**: `''`

The main title text, supporting for `\n` for newlines.

\[WARNING\]: When enabling [toolbox.feature.saveAsImage](option.toolbox.md#feature.saveAsImage), and [toolbox.feature.saveAsImage.name](option.toolbox.md#feature.saveAsImage.name) is not provided, it has historically been using `title[0].text` instead. This usage is not recommended -- [toolbox.feature.saveAsImage.name](option.toolbox.md#feature.saveAsImage.name) should always be specified explicitly; otherwise, **correctness** and **security risks** for a filename have to be considered in this `title.text` option. See document ["Security Guidelines"](http://localhost/echarts-website/handbook/en/best-practices/security) for recommendations on safe usage.

## link
- **Type**: `string`
- **Default**: `''`

The hyper link of main title text.

\[WARNING\]: This URL string is accepted directly without any internal sanitization. **Security risks** must be considered if it comes from untrusted sources. See document ["Security Guidelines"](http://localhost/echarts-website/handbook/en/best-practices/security) for recommendations on safe usage.

## target
- **Type**: `string`
- **Default**: `'blank'`

Open the hyper link of main title in specified tab.

**options:**

*   `'self'` opening it in current tab
    
*   `'blank'` opening it in a new tab

### textStyle.color
- **Type**: `Color`
- **Default**: `'#333'`

main title text color.

### textStyle.fontStyle
- **Type**: `string`
- **Default**: `'normal'`

main title font style.

Options are:

*   `'normal'`
*   `'italic'`
*   `'oblique'`

### textStyle.fontWeight
- **Type**: `string|number`
- **Default**: `'bolder'`

main title font thick weight.

Options are:

*   `'normal'`
*   `'bold'`
*   `'bolder'`
*   `'lighter'`
*   100 | 200 | 300 | 400...

### textStyle.fontFamily
- **Type**: `string`
- **Default**: `'sans-serif'`

main title font family.

Can also be 'serif' , 'monospace', ...

### textStyle.fontSize
- **Type**: `number`
- **Default**: `18`

main title font size.

Link resolution

Internal links in HTML (href="#property.path" and href="(option|api|tutorial).html#...") are resolved to relative .md file paths before turndown conversion on a best-effort basis.
Out of ~22,600 total links in the source, ~12,000 internal links are resolved to .md file paths (~99%), ~10,600 external links are preserved as-is, and ~50 links with non-standard formats (e.g. missing # prefix) are left unresolved.