Add docs for configuring committer name/email for Gitaly
What does this MR do and why?
This configuration has been introduced in Specify GitLab as a Committer for commits creat... (gitaly#5715 - closed). We need it to set committer
of the commits created by Gitaly to GitLab <noreply@gitlab.com>
. In this case, a user can specify noreply@gitlab.com namespaces="git" ssh-ed25519 ...
in the ~/.ssh/allowed_signers
file and their commits will be verified locally.