docs: refresh README

[gordonmurray]

The README has drifted from the code in a few places:

• "Legacy vectors: pa.list_(pa.float32(), dim) - schema only, may show corruption warnings" is outdated since feat/fix: Support for FixedSizeList vectors, nested schemas, and UI text wrapping #14 and fix: replace hardcoded "images" dataset check with generic corruption handling #40. Variable-length float lists serialize fully (stats, sparklines) on all six supported Lance versions; the test suite added in test: add API endpoint test suite and enable the CI test job #50 covers this.
• The version table says the 0.29.2 image ships PyArrow 21.0.0, but its constraints file allows >=16,<22.
• "Large vectors: >2048 dimensions show preview only" — there is no 2048 threshold in the code; every vector cell shows a 32-value preview regardless of dimension.
• The tip recommends starting with lancedb-0.24.3 while the quick start and version table recommend 0.29.2. The docker-compose example also still uses 0.24.3.
• The stable, latest, v{app version} and app-{version}_lancedb-{version} tags from chore: centralize app version and publish release artifacts #46 are not documented, so there is no visible way to pin to an app release.
• Nothing mentions the test suite from test: add API endpoint tests #28 or how to run it.
• No links to CONTRIBUTING.md or CHANGELOG.md.

Plus minor cleanup: stray blank lines before the title and some wording that does not match the tone of the rest of the docs.

One PR to fix all of the above.