docs / comparison: mention GPG Signed Commits?

[Mikaela]

I noticed that https://docs.gitea.io/en-us/comparison/ under code management ( https://github.com/go-gitea/gitea/blob/main/docs/content/doc/features/comparison.en-us.md#code-management ) mentions GPG Signed Commits.

I don't think anything else than Gitea supports SSH Signed Commits yet (at least not GitHub or GitLab) being so recent feature, I think it could be nice to mention.

[lunny]

@Mikaela Would you like to send a PR to add that?

[lunny]

@Mikaela Would you like to send a PR to add that?

I did it. :)