gh-142466: Pixi package definitions for downstream use (#142469)

* WIP: ENH: Pixi package definitions for downstream development

[skip ci]

* linux-64 support

* tidy gitignore

* respond to review
- switch cases on `PYTHON_VARIANT`
- remove `minor_version` by using `python3`
- remove runtime-only asan options

* README updates

* use `.md` to preview rendering

* Apply suggestions from code review

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

* Apply suggestion from @FFY00

Co-authored-by: Filipe Laíns <filipe.lains@gmail.com>

* Apply suggestion from @FFY00

Co-authored-by: Filipe Laíns <filipe.lains@gmail.com>

* Apply suggestion from @FFY00

Co-authored-by: Filipe Laíns <filipe.lains@gmail.com>

* Apply suggestion from @lucascolley

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Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Co-authored-by: Filipe Laíns <filipe.lains@gmail.com>