Add more extensions to Markdown rendering

* Admonitions: https://www.npmjs.com/package/marked-admonition-extension
* Footnote support: https://www.npmjs.com/package/marked-footnote
* Custom heading IDs: https://www.npmjs.com/package/marked-custom-heading-id
* Code highlighting (already have this added, but perhaps this library will be a little less jank) https://www.npmjs.com/package/marked-highlight