Reorganize service for gitlab-pages

I missed it during the review but there's something confusing with how the gitlab-pages service is set up.

So our goal is we want to configure sessionAffinity on the gitlab-pages service.

AFAICS we achieve this with:

• file chart/templates/gitlab-pages-service.yaml which looks like it duplicates the upstream gitlab-pages service https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/charts/gitlab/charts/gitlab-pages/templates/service.yaml
• new Helm value gitlab.serviceName which inserts itself in the middle of the upstream chart's Helm values.

But I see a couple of issues:

• there is no explanation at all why we duplicate the gitlab-pages service and where it was copied from. Sooner or later there will be changes to the service in the upstream chart that will require us to update the duplicate too. This must be thoroughly explained in the Helm template and we need a warning about this in the procedure to update the gitlab chart version (see also #8 about that)
• The Helm value gitlab.serviceName is misplaced. It is not actually a value for the gitlab subchart, and the template using this value is defined at the top-level of the gitlab-pages-authenticating-proxy chart. I suggest to rename this value to sth like gitlabPagesSessionAffinityServiceName (outside gitlab) and put again some explanations next to the Helm value regarding why we need this and what it does.