Amend Omnibus source mirror documentation
Amend Omnibus source mirror documentation

- Updates the documentation around adding git repository mirrors of
  components in Omnibus for new defaults in the User Interface
- Updates the documentation to have the mirror on Dev created by the bot
  user which has the appropriate permissions.

Related https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/3882

Signed-off-by: Robert Marshall <0465b3b4f3ae0103ad50d09ff86a3523700a2979@gitlab.com>

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab