Remove justfile & fully replace remaining commands with pixi

[Wumpf]

What

• Fixes Remove justfile #5595

Has a few judgment calls on what is no longer needed, but preserves most of what we had.
Notably removes py-requirements command for the time being - the hope is that #5890 will fix things. (porting it proved difficult)
See

• Re-establish example requirements.txt dependency check #5894

Other things in here:

• taplo (toml formatting) was very slow since despite extensive 'exclude' list it looked at too many files. Turned it around to 'include' things and it's a LOT faster now
• Updated Ruff & removed last use of blackdoc. The only thing we're not properly catching with this now is too long lines, I figured it's not worth keeping it around just for that.
• various cleanups of pixi.toml file: trying to make commands a bit more consistent overall

Tested on:

• mac
• windows

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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[emilk]

Fan-fucking-tastic!