docs: Explain how to set up a personal gitlab runner.

I'm not the only one doing it, so document it, especially since there's a
new trick as of
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5669

Reviewed-by: Andres Gomez <agomez@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5988>
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