Setting up the documentation

[NoraLoose]

This PR sets up a sphinx-based documentation that supports both markdown / MyST and ReStructuredText (RST).

Notes

1. I moved copied the notebook cstar_example_notebook.ipynb from the examples directory to the docs directory so that we can include that notebook in the documentation. (It can't sit outside the docs directory.) I removed the uppermost markdown cell with the caption "Installation" from the notebook because
   • we have a separate installation section in the docs
   • both "Installation" and "Introduction to C-Star “v0” with the roms_marbl_example Case" are top-level headings ("#"), but sphinx wants there only to be one top-level heading, otherwise the formatting is off
2. I added the sphinx-based dependencies to ci/environment.yml so that we can test the documentation build as part of the CI
3. I added a test that builds the documentation to the GitHub workflows

Still to do in future PRs

• [x] Hook the documentation up with ReadTheDocs (RTD)
• [ ] Reformat some of the docstrings (there are lots of warnings raised when building the API reference). This should be handled by pre-commit, so I suggest adding pre-commit to the CI and cstar_env. (I'm happy to do this in a follow-up PR).
• [ ] Refactor tests to use pytest because that's what the Contributor Guide (docs/contributing.md) promises.

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[NoraLoose]

@dafyddstephenson does either 1. or 2. in my notes above raise any red flags for you?

[NoraLoose]

@dafyddstephenson does either 1. or 2. in my notes above raise any red flags for you?

If not, I suggest to merge so that I can test whether RTD can successfully deploy the documentation.

[NoraLoose]

There is still some work to do for future PRs (I will document these in issues), but this is good to merge for now.