Add immediate instructions for commit signing failure

[yizha1]

Is your feature request related to a problem?

Recently, new contributors have been missing the step to sign commits during PR creation, as seen in these instances:

• test: fix build on 32bit platforms notation-go#500
• feat: helper function in ioutil to PrintObjectAsTree #1129

To streamline this process, it's beneficial to provide immediate instructions when an error occurs during PR creation, enabling users to fix the issue right away.

Although the contributing guide outlines the requirement for signing commits, it's not easily accessible for users to find and follow promptly.

What solution do you propose?

We may try this GitHub actions https://github.com/marketplace/actions/check-signed-commits-in-pr and use comments for instructions of signing commits.

What alternatives have you considered?

N/A

Any additional context?

No response