doc: Update Gitaly documentation to use new way to configure Git

What does this MR do and why?

Gitaly has deprecated usage of gitconfig files and will start to unconditionally ignore them with v16.0. Our GitLab Charts and Omnibus have both already been adapted to ask Gitaly to ignore gitconfig files, which means that any setting set in them will not be effective.

Update our documentation to recommend the new way of doing things:

- For source installations, any Git configuration needs to be added
  via `[[git.config]]` sections to Gitaly's `gitaly.toml`.

- For Omnibus, users are asked to instead use `gitaly['gitconfig']`.
  Note that settings configuration via `omnibus_gitconfig['system']`
  still works right now as any non-default values configured in it
  will be used as default value for `gitaly['gitconfig']` in case
  the latter is not set. This fallback will be removed with v16.0
  though.

MR acceptance checklist

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