Document mathematical notation

[khaeru]

Following on the 2024-03-28 MESSAGE meeting (#388 (comment)), this PR proposes a style of mathematical notation to be used in the docs.

This style is:

• Intended to be more explicit and easier to read and edit than the existing style.
• Documented on a new page.
  The previous/extant style was not documented fully or in any user-visible location.

This PR is not intended to include revision of all equations to match the style, only to establish it. That revision would be accomplished in 1 or more follow-up PRs by a team of contributors.

How to review

• Look at the ‘Notation’ page in the preview build of the docs.
• Comment below.

PR checklist

• Discuss and implement feedback.
• Complete the section on LaTeX source.
• Maybe expand the Sphinx LaTex/MathJax config.
  In particular, we could define LaTeX commands like \output to expand to \text{output}_{c h h^D l m n^D n^L t y^A y^V}, since these are almost always written the same way. This would make the LaTeX source of the equations much easier to read and edit.
• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.4%. Comparing base (1fca82f) to head (f5f7ff6).
Report is 1 commits behind head on main.

Additional details and impacted files

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