fix(docs): create placeholder section for python-docs to preserve navigation structure

[devin-ai-integration]

Description

Refs: Related fern-platform PR: fern-api/fern-platform#6604

Fixes the navigation issue where the sidebar disappears when entering Python library docs API reference pages. The CLI now creates a placeholder SectionNode at the exact position where python-docs is defined in the navigation YAML, which FDR can find by slug suffix match and replace with the actual library docs content.

Link to Devin run: https://app.devin.ai/sessions/12c061bbb9af4494ad68657c7285342b
Requested by: paarth@buildwithfern.com

Changes Made

• Changed PythonDocsSectionHandler return type from PageNode | null to SectionNode | null
• Updated createPythonDocsSectionPlaceholder to return a SectionNode with empty children array
• Added pythonDocsSectionHandler in publishDocs.ts that creates a placeholder section during the publish flow
• The placeholder section preserves the navigation position so FDR can inject library docs at the correct location

Testing

• Unit tests added/updated
• Manual testing completed (requires fern-platform PR to be deployed together)

Human Review Checklist

• Verify the SectionNode structure has all required fields (id, type, slug, title, children, etc.)
• Confirm placeholder ID pattern __libdocs_placeholder_${urlSlug}__ matches FDR expectations in fern-platform#6604
• Review cross-repo coordination: this PR must be deployed alongside fern-platform#6604
• Verify slug suffix matching will work correctly (FDR looks for sections where slug.endsWith(libraryDocsSlug))

[devin-ai-integration]

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