Docs: add signed commit guidance (#93155)
## What?
Adds contributor-facing guidance for the new signed-commit requirement.
- adds a `Signed commits` section to `contributing.md`
- adds a short signed-commit note to the PR template
- clarifies that a `Signed-off-by` line is not enough
## Why?
As signed commits become required, contributors need a clear explanation
in the repo and at PR time so they know what will be enforced and how to
recover from unsigned commits.
## How?
Documents the requirement in the main contribution guide and repeats the
key action items in the PR checklist.
## Testing
- docs-only change
- verified with local readback and diff review
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