build-docs: Add sphinx-autobuild for live reloading

[andersk]

Testing plan: tools/build-docs --watch, then view http://localhost:9995.

[zulipbot]

Heads up @andersk, we just merged some commits that conflict with the changes your made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.