GitLab CR installation docs are out of date

With the merge of https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/merge_requests/86, https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/blob/master/doc/installation.md is out of date and needs a revisit. We should recommend users to deploy the CR the same way we deploy for testing.

AC:

• https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/blob/master/doc/installation.md documents how to run https://gitlab.com/gitlab-org/gl-openshift/gitlab-operator/-/blob/master/scripts/test.sh
• Consider renaming test.sh to install-gitlab.sh or similar, to make its purpose more clear.