doc: Document how to generate Gitaly configuration on startup

Patrick Steinhardtrequested to merge
pks-docs-gitaly-document-config-command into master

What does this MR do and why?

Gitaly has recently introduced a new feature that allows an admin to configure parts of Gitaly configuration on startup via an external command. This MR documents this new feature.

See https://gitlab.com/gitlab-org/gitaly/-/issues/4828+ for the issue, gitaly!5525 (merged) for the implementation.

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