Docs: Specify Gitlab public key for commits signing

What does this MR do and why?

Within Automated/web commits (merge or web IDE commits... (#19185 - closed) web commits signing has been introduced. A private key is configured in Gitaly. The private key is then used to sign the commits created in UI.

The users may want to verify the commits signed by GitLab locally. They need the public part of the private key for it.

Let's hardcode it in the docs for now and create an endpoint to fetch the up-to-date value in a follow-up