Add `uv` / `uv_build` to the Python Packaging User Guide

[johnthagen]

uv and uv_build are not mentioned in the Python Packaging User Guide. For example at:

• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#declaring-the-build-backend
• https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license-and-license-files

It would be helpful if Astral contributed to that guide so that uv-related tools were highlighted.

[konstin]

I think that's roughly the same as #14672, and the PR at pypa/packaging.python.org#1880.