Add reproducible pixi docs and test workflows

[jdbcode]

Summary

Add reproducible Pixi-based development workflows for tests and docs, and document the recommended contributor setup.

What changed

• Pin Pixi Python to 3.12.* to avoid unsupported resolver/build paths during environment setup
• Separate Pixi solve groups so the tests environment does not co-solve with heavier dataflow dependencies
• Add a dedicated docs Pixi feature/environment with Sphinx and related doc build dependencies
• Add Pixi tasks for docs build and strict docs checks
• Commit pixi.lock so contributors get reproducible environments across supported platforms
• Update contributor guidance to recommend Pixi for local setup and pre-PR checks
• Clean up docs debug output so builds no longer assume pip is installed in the docs environment

Validation

• pixi install -e tests
• pixi run -e tests pytest -q xee/ext_test.py
• pixi run -e tests pytest -q xee/ext_integration_test.py -k 'test_open_dataset'
• pixi install -e docs
• pixi run -e docs docs-build
• pixi run -e docs docs-check

Why

This makes docs building and test running reproducible for contributors without turning development-only tooling into package extras for end users.