Update docs

[kboone]

This contains a big update to the Spectrum docs, and bunch of small fixes to the rest of the docs for things that have broken over time.

[benjaminrose]

Thanks @kboone! I'm appreciative that someone found time to work on the documentation.

Two related questions: 1) Once this is merged, should we start a release with SALT3 and these changes? 2) Is there something we can do to ensure future model additions don't break the "List of Built-in Sources"?

[kboone]

To avoid this in the future, we should add a test to make sure that the docs compile without errors. There were errors in that build, but they weren't flagged when we were merging PRs.

Yes, I'll put out a new release when this is merged.