test: enforce release notes index ordering

[X-PG13]

Summary

• add release index parsing coverage for English and Simplified Chinese release notes indexes
• verify Latest points to the current package version
• verify Recent Releases follows the previous three CHANGELOG versions in descending order

Validation

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

Closes #140