add Table of contents to documents

[horacioh]

it would be nice to get a view of the editor table of contents that list all the headings in the document

• [ ] it should be a new editor mode
• [ ] it should have links so users can navigate to headers
• [ ] if should be sticky to the page

Examples

https://docusaurus.io/docs/markdown-features/toc

[ericvicenti]

We have TOC in the site and it would be pretty easy to port it to the app's publication page. The editor would be a bit more tricky but is certainly doable