Update doc/administration/pages/index.md to mention the non-support of subgroups

Originally submitted at
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5734.
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/administration/pages/index.md

@@ -11,6 +11,7 @@ description: 'Learn how to administer GitLab Pages.'
 - This guide is for Omnibus GitLab installations. If you have installed
   GitLab from source, follow the [Pages source installation document](source.md).
 - To learn how to use GitLab Pages, read the [user documentation][pages-userguide].
+- Does NOT support subgroups. See [this issue](https://gitlab.com/gitlab-org/gitlab-ce/issues/30548) for more information and status.
 
 This document describes how to set up the _latest_ GitLab Pages feature. Make
 sure to read the [changelog](#changelog) if you are upgrading to a new GitLab