Follow Sphinx style guide in documentation rst files

[SJaffa]

When editing the documentation served at https://docs.matflow.io/stable/ and https://hpcflow.github.io/docs/stable/, the text files use rst (reStructurdText) format. We should follow the Sphinx documentation guide for sections/headings. This seems to be a bit inconsistent between files (see xamples below), so should we add a GitHub Action or precommit hook to enforce it, e.g. sphinx-lint?

Examples:
In the MatFlow docs homepage using = underline for top-level headings.

In the Matflow docs contribute page using # underline for top-level headings, then = underline, then - underline.

In MatFlow docs environments page, using # underline, then ~ underline, then - underline.

The Sphinx docs recomments this order:

• # under- and overline
• * under- and overline
• = underline
• - underline
• ^ underline
• " underline

[SJaffa]

We're using this problem-matcher to show inline rst errors in PRs (cool thing I've not seen before!), but it doesn't seem to catch the headings. It looks like we're not using it in the recommended way so we might be missing updates, should we change to this?

[gcapes]

I think this would be a good (but agreed, low priority) thing to do. I don't think it matters to rst/sphinx as it infers the heading level from context so long as the markup is consistent within each file, but for readability it would be an improvement.

[dkfellows]

The Sphinx docs recomments this order:

# under- and overline
* under- and overline
= underline
- underline
^ underline
" underline

That should probably be treated as HTML <h1>...<h6>, where the recommendation is to only put at most one <h1> in an article-sized document. (A book would be different.)