Add HTTP resources to TOC

[sevdog]

This PR adds the methods to handle TOC (table of content) generation which can be used by themes like furo to build a page-level-toc.

This can simplify nagivation in pages when there are many HTTP endpoints.

[stevepiercy]

@sevdog does this generate the right-hand "On this page" navigation, such as this in Furo?

If so, how is that different from the current implementation at https://sphinxcontrib-httpdomain.readthedocs.io/en/stable/index.html?

To merge this PR, it needs the following:

• rebase on main
• change log entry into doc/changelog.rst
• documentation in doc/index.rst about what this does or supports differently than the current implementation

Please let me know. Thank you!