build/pkgs/*/distros/pyodide.txt: New, corresponding to pyodide 0.25.0

[mkoeppe]

We add files that cross-reference the non-Python library packages that are available in the Pyodide distribution (https://github.com/pyodide/pyodide/tree/main/packages).

For now, this is just documentation.

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 6223e27; changes) is ready! 🎉

[kwankyu]

OK.

I don't know how pyodide platform could be useful to sage users now. But I guess this is a preparation for the future.

[mkoeppe]

Thanks!