ci: migrate to pixi

[maximlt]

This PR migrates the developer workflow to Pixi. This is pretty much aligned with what has already been done for most HoloViz packages, with some differences:

• Adopted recommending using the default environment with pretty much all that is required to develop hvPlot (tests, jupyterlab, etc.), excluding building the docs (could be included). This approach was recently adopted by HoloViews. EDIT: With a bit more streamlined experience provided by pixi run install-dev (create env, editable install, set up pre-commit, download test datasets).
• It didn't move yet to hatchling to build hvPlot as I don't want to mix things too much in this PR that is already big enough.
• It doesn't include the "Waiting room" in the GH build workflow, my experience releasing Param was that I always forget clicking on the button to publish and I didn't see much benefit using it.
• It keeps the extras in pyproject.toml and some jobs running pip, I'd like to keep the option to develop hvPlot with pip (even if undocumented), I'm concerned Pixi makes the life of re-distributors too difficult (making it less likely for hvPlot to be re-distributed).

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• Add PR description
• Update pixi.toml to use new approach adopted by HoloViews (1 working dev env)
• Ensure the tests / docs / build workflow work as expected
• Update the developer guide