docs: convert index.rst to MyST .md

[Jammy2211]

Summary

Converts the single prose page (docs/index.rst, a thin wrapper around the project README) to MyST Markdown for consistency with the conversion across the other PyAuto libraries. Adds myst_parser to the Sphinx extensions list — it wasn't previously configured because the docs were RST-only.

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

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 12 commits since the conversion (numpy-2.x __array__ fix, sparse-CPU OOB fix, interferometer pixelization plot fix, jax cleanup, etc.). None of those commits touched the only converted file (docs/index.md), so the merge was clean — no conflicts. See merge commit 9100e0b.