DOC: Improve contrib instructions for Pixi #1570

Job	Run time
cache-pixi-lock / Generate output	45s
Unit tests: ubuntu \| pixi run -e test-py311 tests	2m 31s
Unit tests: ubuntu \| pixi run -e test-latest tests	2m 35s
Integration: ${{ matrix.os }} \| pixi run -e ${{ matrix.pixi-environment }} tests-notebooks	0s
TypeChecking: pixi run typing	0s
merge-test-artifacts	0s
	5m 51s

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