Document mathematical notation

[khaeru]

Following on the 2024-03-28 MESSAGE meeting (https://github.com/iiasa/message_ix/pull/388#issuecomment-2024738439), 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 ✅
• [x] Add, expand, or update documentation.
• [ ] Update release notes.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #811 +/- ## ===================================== Coverage 95.4% 95.4% ===================================== Files 46 46 Lines 4354 4354 ===================================== Hits 4156 4156 Misses 198 198 ```