Convert RST docs to markdown

[maxrjones]

This PR changes the docs structure to be based on markdown, still via sphinx, using the myst-parser. The goal is to improve maintainability.

Closes #828

TODO:

• Unit tests and/or doctests in docstrings
• Tests pass locally
• Docstrings and API docs for any new/modified user-facing classes and functions
• Changes documented in docs/release.rst
• Docs build locally
• GitHub Actions CI passes
• Test coverage to 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.33%. Comparing base (456a3e1) to head (a26bbbe).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #830   +/-   ##
=======================================
  Coverage   99.33%   99.33%           
=======================================
  Files          26       26           
  Lines         898      898           
=======================================
  Hits          892      892           
  Misses          6        6           

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