Revise PR titles and bodies with runner cache guidance

[CasperKristiansson]

Summary

Update generated PR titles and bodies to call out runner cache availability and link to actions/setup-python guidance, giving reviewers actionable context before merging.

Why

Maintainers often need to confirm runner support and cache behavior. Including that information directly in the PR lowers friction and prevents broken merges.

What needs to happen

• Enrich PR titles/bodies with runner availability details (leveraging existing or new runner checks).
• Add links to relevant actions/setup-python docs or tutorials for warming caches.
• Keep body content configurable so existing consumers relying on structured sections continue to work.
• Update tests or snapshots for PR content to cover the new sections.
• Document the change in README or CHANGELOG.

References

• Roadmap policy item: "Split PR titles/bodies".
• https://github.com/actions/setup-python

Acceptance criteria

• Generated PRs include runner availability notes and setup-python references.
• Snapshot tests cover the new messaging to prevent regressions.
• Documentation highlights the additional context provided.