Add an action-broke troubleshooting checklist to the docs

[Metbcy]

The issue template asks users what they ruled out, but the docs should give the same checklist before they file an issue.

Scope:

• Add a troubleshooting subsection to docs/src/github-action.md.
• Cover missing pull-requests: write, non-PR events needing explicit refs, missing path, and verify-signatures failures.
• Link to the action-broke issue template at the end.

Definition of done:

• mdbook build docs succeeds.