Update python readme and add py-wheel command

[Wumpf]

What

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: rerun.io/viewer
  • Using examples from latest main build: rerun.io/viewer
  • Using full set of examples from nightly build: rerun.io/viewer
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Recent benchmark results
• Wasm size tracking

[abey79]

There are imho 3 building "mode" for dev/motivated users, in that order of importance:

• the standard dev way of thing (aka pixi)
• build/install from source in some venv in editable mode (pip install -e rerun_py/.)
• build custom wheels (presumably to share them around, or cache them to install in multiple place) (pixi run py-build)

Ideally the docs should reflect that. I'd put the first 2 in readme, and cover the 3rd only in BUILD.md