Update Sphinx, Theming, and Faster Builds

[vector-of-bool]

This changeset does a few things:

1. Sphinx is now updated to 7.0.1
2. The vendored theme has been removed, and documentation is now generated with Furo
3. Generated documentation uses the dirhtml Sphinx builder. This generates "prettier" URLs by placing every page within its own directory with a single index.html inside. The result is that URLs no longer have a .html suffix.
4. To keep old links working, the old .html files are generated and just redirect to the new URLs.
5. The "man pages hack" that we were using was breaking parallel builds in Sphinx. This hack has been replaced with a less-janky hack that works with parallel builds.

Other than the above, the documentation content has remained unchanged. Future changes will include documentation content updates. The rendered results of these changes are available in the make-docs tasks in EVG.

[eramongodb]

Exciting changes! Minor feedback; otherwise LGTM.

[kevinAlbs]

LGTM. I like the new documentation theme. And the parallel build is much appreciated.