Documentation Updates

[MelissaAutumn]

It's spring, and our documentation could use a little spring cleaning.

This is a bit of a meta-issue for documentation

• [x] Fix readthedocs
• [ ] Update AMO references to ATN
• [ ] Verify that the API actually works the way the documentation describes
• [ ] Verify the docker install process works

[MelissaAutumn]

Readthedocs is now fixed: https://thunderbird-addons-server.readthedocs.io/