[Feature] Add ReadTheDocs to CI

[bmhan12]

Make ReadTheDocs build as part of our CI checks.

ReadTheDocs does have a feature so you can trigger RTD builds on pull requests:
https://docs.readthedocs.io/en/stable/pull-requests.html

It's a toggle on/off with the option to make PR builds private or public to everyone.
I think we want public, so the PR author can view the build.

@rrsettgast, I have admin access, should I flip it on?

[rrsettgast]

Yes!!!

[bmhan12]

RTD builds should be on for PR's now.

[TotoGaz]

Will it deploy for all PRs as well? This may not be what we want?

[klevzoff]

@rrsettgast @bmhan12 I got a failed check in #1847, but when I click it I get a 404 page from readthedocs. It would be nice to have the ability to see the error log and re-trigger the build, but I imagine it's impossible without giving everyone RTD admin access?