Write a message in PR when is mentioned in a Discussion

[oriolpiera]

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Like it happens in Issues of any repository, it will be great that when a PR is mentioned in a Discussion, in the PR itself appears a reference that this PR was mentioned in a Discussion, and a link to it Discussion.

Example when a PR is mentioned in a Issue:

[oriolpiera]

I close this discussion. I didn't see related ones:

• https://github.com/orgs/community/discussions/2971
• https://github.com/orgs/community/discussions/3211