Create a standalone doc for PSR (Python Semantic Release) commit expectations

[farhan]

Summary

OSPR creators need clear guidance on commit expectations. We should capture the details and relevant references in one standalone doc.

Proposed work

Create a docs.openedx.org page covering:

• How Python Semantic Release (PSR) derives the version from commit history, so contributors never bump versions manually.
• How PSR parses commit messages to pick the version bump and build the changelog.
• The Conventional Commits mapping to patch, minor, and major releases, following OEP-0051.
• Why commit messages must be accurate: PSR reads them to generate release notes and changelogs.
• How to signal a breaking change and where to document it.
• It will be great if the PR is squashed when pushed for the final merge. (Nice to have)

Reference the real guidance from xblocks-extra#29: version comes from the SCM tag and breaking change → major bump.

Acceptance criteria

• Standalone doc published on docs.openedx.org covering the above.

Follow-up: #567 links this doc from OSPR templates.