docs: recommend entering the pixi shell

[winstxnhdw]

📝 Summary

Last week, I had an issue where pnpm was hitting OOM errors when installing github-copilot-language-server. This blocked me from building marimo entirely. I initially thought this was an issue with pnpm and went down a deep rabbit hole. Eventually, I found out that the issue was actually due to a bug in Node v24.4.0. The week long debugging session of pain could've been avoided if I had remembered to enter the pixi environment and used the environment's node binary.

// cc: @nojaf

📋 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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Name	Status	Preview	Comments	Updated (UTC)
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[mscolnick]

I think this change was included in #6679, so im going to close this PR.

Let me know if there are other tweaks to clear up the docs for contributing with pixi. thanks!