docs: convert prose .rst files to MyST .md

[Jammy2211]

Summary

Converts 30 prose documentation pages from reStructuredText to MyST Markdown so they're easier for contributors and AI tools to read and edit. The autosummary-driven docs/api/ and docs/_templates/ files are intentionally left as .rst — their dense directive blocks gain nothing from a MyST colon-fence rewrite, and the autogenerated _autosummary/ stubs are .rst regardless.

Adds dollarmath to myst_enable_extensions in docs/conf.py for the existing math blocks in overview/the_basics.md and science_examples/astronomy.md.

myst-parser was already configured. Sphinx accepts mixed .rst/.md source trees, and toctree entries reference file stems, so no toctree edits were needed.

Part of #1245.

API Changes

None — internal docs only.

Test Plan

• Read the Docs build passes
• Local sphinx-build -b html docs docs/_build/html runs cleanly with zero new warnings on converted .md files

Full API Changes (for automation & release notes)

None — this PR only changes documentation source files.

🤖 Generated with Claude Code

[Jammy2211]

Branch caught up to main.

main advanced 13 commits since the conversion — most are unrelated source/test changes (autoarray-side jax cleanup, EP cavity-message factor, etc.). The relevant docs-touching commits were the two automated 2026.5.1.1 and 2026.5.1.4 Colab URL-tag bumps, which modified docs/index.rst (which the conversion deleted). The modify/delete conflict was resolved by keeping the deletion and porting the bump (2026.4.13.6 → 2026.5.1.4) to the MyST docs/index.md sibling. See merge commit 2cc8701.

No structural changes — URL-tag substitution only.