CI, DOC: Separate docs build from CI test jobs, deployment

[nray]

• Fixes DOC, CI: only need to build/lint docs once in the CI #32
• Fixes DOC, CI, MAINT: deploying docs #33

This PR does the following:

1. Separate docs run from the base CI.
2. Separate docs deployment from the other CI runs.

For deployment, github action is used and the deployed page is at https://lanl.github.io/GFDL/.

Github pages restrict deployment from PR's, as a work-around, created a "test-docs-deployment" branch and added that to the list of branches from which pages can be deployed (Settings->Environment->github-pages->Deployment Branches and Tags) .
The deployment CI job currently would be invoked for push events to the main and the test branch.
To invoke a deployment, the test-branch has to be kept in sync with the PR branch.

What is left to do:

• Re-test the deployment.
• Remove the test branch from the deployment config, and merge this PR, delete test-branch.
• The merge should invoke a deployment job from the main repo, and the docs should now correspond to the main branch. Ensure, links are working correctly there. Also, once the PR is merged, we should be able to see a "Run Workflow" button for the deploy job from the UI. That should help with launching the deployment manually.

Further things to do (not necessarily as part of this PR) :

• Deploy docs corresponding to released tags (this seems doable with github actions), however, we need to make sure that the links to the code source's from the method api's should point to the correct tagged version of the code.
• Explore using circleci. Circleci, to my understanding, is another CI/CD framework, many existing codes use it, including scikit-learn. And a brief search says that it can support deployments from PR's. So, definitely worth further exploration.

[tylerjereddy]

I've edited the description based on which issues this WIP PR aims to address. Feel free to adjust if I got them wrong. Keeping them linked like that makes it easier to see which issues are already being worked on.

[nray]

@tylerjereddy : Ready for final review. The circle ci / tag-specific deployments would be in a separate PR.

[tylerjereddy]

Thanks -- I just made one suggestion, rest seems "ok" modulo test-docs-deployment branch cleanup stuff.

[tylerjereddy]

gh-32 suggests that we only need to lint the docs in one CI job, so we can delete this part of the CI run now, right?

[nray]

Fixed.

[nray]

Thanks -- I just made one suggestion, rest seems "ok" modulo test-docs-deployment branch cleanup stuff.

Sounds good, addressed your comment and removed the test-branch from deployment. Go ahead and merge this PR when the CI passes. I will delete the branch once I ensure that deployment from main happened as expected.