Feat(doc): docs on Gitaly GPG signing

What does this MR do and why?

This MR documents the new Gitaly feature: GPG signing of commits made using GitLab interface.

• Issue: #19185 (closed)
• Gitaly MR: gitaly!4802 (merged)
• Omnibus MR: omnibus-gitlab!6294 (merged)

MR acceptance checklist

• Finalize and accept !95231 (closed)