Request: TOC for doc pages

[nynexman4464]

The CLI doc is hard to navigate. I wish it had a table of contents or something so I could scroll to the command I'm interested in.

https://astryx.atmeta.com/docs/cli

[nynexman4464]

Actually I'd like to see this for all the doc pages - https://astryx.atmeta.com/docs/styling for example could benefit from this also. #3081 is also probably required to do this (EG linkable sections)

[cixzhang]

@ernestt was working on adding Outline to some of these pages. Maybe we can extend it to the Markdown ones that are derived from our README files.

[ernestt]

Addressed in #3101.

Documentation pages now render an "On this page" table of contents — a sticky outline on desktop and a "jump to section" selector on mobile — including the CLI/library README pages. Section headings also got stable ids so the TOC entries deep-link correctly.