Migrate docs to ReadTheDocs

[ryanjjung]

Thunderbird has a ReadTheDocs subscription, and we have precedent with other projects publishing docs there. tb_pulumi currently has a custom-ish Sphinx build and a Github Action workflow to publish them to Github Pages. This is extremely low priority, but we might consider publishing to ReadTheDocs at some point in the future.

[MelissaAutumn]

Here's a sample .readthedocs.yaml file:
https://github.com/thunderbird/thunderbird-accounts/blob/main/.readthedocs.yaml

And my sphinx conf.py for any extras:
https://github.com/thunderbird/thunderbird-accounts/blob/main/docs/conf.py

in my case I use uv for the project, so I need to generate a lockfile, but you should be able to remove that section if we're just using pip here.