Fix build_docs github action + some minor improvements

[ben-albrecht]

Fix build_docs github action that was previously broken. Some other minor improvements were added as part of this fix.

Changes include:

• build_docs action is triggered when we push to develop branch
  • Previously, it was on PR closed, which isn't necessarily a merge, i.e. a contributor could close a PR without merging and the action would still trigger
• Use GITHUB_TOKEN instead of personal authentication (PA) credentials to push
• Use "action@github.com" as the git author (user) to distinguish automated commits from human commits
• Check static files from doc branch into develop, so that we can modify
  them in a regular PR to develop, rather than have to edit the doc branch
  directly. I'd like for us to get to a point where doc branch pushes are fully automated, so we should never have to modify it directly.
  • Static files include: .nojekyll (both top-level and per-sphinx-build), CNAME, and top-level index.html

[ben-albrecht]

Example build log here:

https://github.com/ben-albrecht/SmartSim/actions/runs/1824864337

This builds the docs from develop branch and deploys them to:

https://ben-albrecht.github.io/SmartSim/develop/

The master docs that were already checked into the doc branch are still visible as the top-level index.html:

https://ben-albrecht.github.io/SmartSim/docs/overview.html

[ben-albrecht]

@Spartee - I noticed this is a different theme than read-the-docs. Is that an intentional change in the recent make docks changes in #133?

If we want RTD theme still, I can take a look at adjusting the requirements-docs.txt

[ben-albrecht]

It looks like there is a separate incompatibility issue in our requirements-docs.txt, but I am not sure if that actually impacts anything other than showing an error during env build:

https://github.com/ben-albrecht/SmartSim/runs/5143975793?check_suite_focus=true#step:5:961

[Spartee]

LGTM

[ben-albrecht]

@Spartee - I noticed this is a different theme than read-the-docs. Is that an intentional change in the recent make docks changes in #133?

For the archives, @Spartee mentioned that this was an intentional change.