Migrate all ReST files to MyST Markdown format

[Copilot]

ReST syntax is more complex to author and maintain than Markdown. This migrates all 9 .rst source files to MyST Markdown (.md), which supports the same Sphinx directives via a cleaner syntax.

Changes

• requirements.txt / environment.yml — add myst-parser
• conf.py — register myst_parser extension, enable colon_fence, update source_suffix to accept both .rst and .md
• 9 RST → MD conversions:
  • Raw HTML blocks: .. raw:: html → ```{raw} html
  • Toctree: .. toctree:: → :::{toctree} (colon-fence syntax)
  • Cross-references: :ref:\text `→{ref}`text ``
  • Label targets: .. _label: → (label)=
  • RST grid/list tables → GFM tables or :::{list-table} directives
  • Hyperlinks: `text <url>`__ → [text](url)
  • Inline code: ``code`` → `code`
  • Zenodo badge table (cite page): RST grid table with nested image directives → GFM table with [![]()]() syntax
• All .rst files deleted

• Fixes Migrate all ReST files to Markdown formats #168

[seisman]

I've built the site locally and it looks good.

[Esteban82]

Works fine for me too.