Modify markdown writing to produce readthedocs pages (#6904)

This commit makes multple changes to the slang doc markdown writer to make the stdlib reference pages usable with readthedocs.
- Adds tables of contents at the bottom of every page with children. TOCs contain category landing pages where applicable and contain uncategorized child pages
- Changes links on the pages to use relative paths instead of absolute paths
- Changes anchor tags to use HTML syntax instead of markdown syntax

The overall TOC on readthedocs will look the same as the TOC seen on the existing docs website, and I have verfified that links and anchors will still work.

The tables of contents generated for use on readthedocs would be visible on the existing docs website.
This change wraps the TOCs in a comment block, so that it will be hidden from view.
The readthedocs build script can filter out the comments to unhide the TOC inside and still properly render it there with it remaining hidden when viewed elsewhere.

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Co-authored-by: slangbot <186143334+slangbot@users.noreply.github.com>

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