docs: Update pip compatibility pages to mention configuration files support (#6410)
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## Summary
Docs at https://docs.astral.sh/uv/pip/compatibility/ still say:
> the future, uv will also support persistent configuration in its own
configuration file format (e.g., pyproject.toml or uv.toml or similar).
For more, see [#651](https://github.com/astral-sh/uv/issues/651).
I think that's done now (?), so updated these to link to
https://docs.astral.sh/uv/configuration/files/
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Co-authored-by: Zanie Blue <contact@zanie.dev>