[2/n] Keep the package README hook in the repository (#3655)

# Summary

This is part of a series to reduce the SDK’s dependency surface where a
small, purpose-built alternative is sufficient.

- Replace `hatch-fancy-pypi-readme` with a small repository-owned Hatch
metadata hook for our existing relative-link rewrite.
- Preserve the README published in package metadata, including when a
wheel is built from the source distribution.
- Reduce the isolated build dependency graph from seven to six
distributions across supported Python versions. Runtime and development
dependencies are unchanged.

## Stack

- https://github.com/openai/openai-python/pull/3653 (merged)
- https://github.com/openai/openai-python/pull/3654 (merged)
- https://github.com/openai/openai-python/pull/3655 👈 this PR
- https://github.com/openai/openai-python/pull/3656