Replace requirements-docs.txt with a python doc pixi environment

[Wumpf]

What

• Part of Replace non-example requirements.txt files with pixi #5895

requirements-docs is still mentioned in python readme, this is handled in

• Update python readme & fix remaining 'just' mentions #5911

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
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[github-actions]

Deployed docs

Commit	Link
3c9a08b	https://landing-qtzyp3ewe-rerun.vercel.app/docs