docs: refresh README

[gordonmurray]

Summary

Brings the README back in line with the code.

Corrections:

• The supported-data-types section said variable-length float vectors (pa.list_(pa.float32())) were schema-only and might show corruption warnings. They render fully since feat/fix: Support for FixedSizeList vectors, nested schemas, and UI text wrapping #14, and the test suite from test: add API endpoint tests #28 exercises them on all six Lance versions.
• Dropped the ">2048 dimensions show preview only" line; there is no such threshold in the code.
• The version table claimed the 0.29.2 image ships PyArrow 21.0.0; its constraints file allows >=16,<22.

Consistency and additions:

• 0.29.2 is now the recommendation everywhere; the tip and the docker-compose example still pointed at 0.24.3.
• Documented the image tags added in chore: centralize app version and publish release artifacts #46 (latest, stable, v{app version}, app-{version}_lancedb-{version}) so users can pin to an app release.
• Added a section on running the test suite locally.
• Linked CONTRIBUTING.md and CHANGELOG.md, and tidied the header.

Test plan

• Checked each claim against the code: vector handling in backend/app.py, the constraints files, and the tag rules in .github/workflows/release.yml
• Test suite instructions are the same commands CI runs, verified earlier on all six constraints files
• Relative links resolve (CONTRIBUTING.md and CHANGELOG.md are at the repo root)

Fixes #51