chore: add marimo editable install in pixi env, simplify docs in CONTRIBUTING.md

[lucharo]

📝 Summary

I realised that the mixed pixi + hatch development set up as advertised in the CONTRIBUTING section was not working as expected. Once activating the hatch shell from within the pixi shell, the pnpm installation was looking outside the pixi env path — see a screenshot below. The new pixi shell + make dev is now a simpler set up

see below a failure to run `make fe-check in the `pixi>hatch` env ❌

and with pixi shell ✅

📋 Checklist

• I have read the contributor guidelines.
• For large changes, or changes that affect the public API: this change was discussed or approved through an issue, on Discord, or the community discussions (Please provide a link if applicable).
• I have added tests for the changes made.
• I have run the code and verified that it works as expected.

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Project	Deployment	Preview	Comments	Updated (UTC)
marimo-docs	Ready	Preview	Comment	Oct 5, 2025 7:36pm