docs: clarify release milestone rollover ownership

[X-PG13]

Summary

• document that the maintainer who merges the release PR or publishes the tag owns milestone rollover
• require the next v1.2.x milestone before follow-up maintenance work accumulates
• keep Configure PyPI trusted publisher and publish ainews-open to PyPI #86 in Deferred: PyPI unless PyPI publishing is explicitly selected

Validation

• ./.venv-dev/bin/python -m unittest tests.test_docs_links tests.test_release_metadata -v
• git diff --check
• make check PYTHON=./.venv-dev/bin/python

Closes #141