Geo - add default prometheus rules for Geo metrics to runbooks

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As a follow up to this issue - #1816 - where we propose adding example prometheus rules to our Geo documentation, it was mentioned here that a longer term action would be to add these rules to our runbooks.

Implementation plan

1. Update our runbooks to include default rules when deploying a GitLab instance as per the discussion here - #1816 (comment 2394064221)
   • Link to runbooks - https://gitlab.com/gitlab-com/runbooks