Link the PSR commit-expectations doc from OSPR templates

[farhan]

Summary

Once the PSR commit-expectations doc exists, link it from the PR templates of all OSPR-accepting repos. Contributors then see the Python Semantic Release expectations before opening a PR, and reviewers point to the doc instead of re-explaining.

Acceptance criteria

• Link to the PSR commit-expectations doc added to the PR templates of all OSPR-accepting repos.

Depends on #566.