docs: convert prose .rst files to MyST .md

[Jammy2211]

Summary

Converts 23 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 substitution to myst_enable_extensions in docs/conf.py for substitution blocks present in the converted prose.

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 PyAutoLabs/PyAutoFit#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 7 commits since the conversion. The two automated 2026.5.1.1 and 2026.5.1.4 Colab URL-tag bumps modified docs/index.rst, all docs/howtogalaxy/chapter_*.rst, and docs/overview/overview_2_new_user_guide.rst — files the conversion deleted. The modify/delete conflicts were resolved by keeping the deletions and porting the bump (2026.4.13.6 → 2026.5.1.4) to the seven matching MyST .md siblings:

• docs/index.md
• docs/howtogalaxy/chapter_1_introduction.md
• docs/howtogalaxy/chapter_2_modeling.md
• docs/howtogalaxy/chapter_3_search_chaining.md
• docs/howtogalaxy/chapter_4_pixelizations.md
• docs/howtogalaxy/chapter_optional.md
• docs/overview/overview_2_new_user_guide.md

See merge commit 8e577a8. No structural changes — URL-tag substitution only.