Add subgroups limitations to Pages docs

GitLab Pages does not support subgroups. We need to add this as Pages limitations.

• https://docs.gitlab.com/ce/user/project/pages/getting_started_part_one.html
• https://docs.gitlab.com/ce/user/project/pages/introduction.html#limitations

cc/ @axil

We have this listed in the subgroups limitations, but it would probably be a good idea to also add it in pages https://docs.gitlab.com/ce/user/group/subgroups/#limitations

created branch 34621-add-subgroups-limitations-to-pages-docs

mentioned in merge request !12637 (merged)

closed via merge request !12637 (merged)

reopened

mentioned in merge request !12891 (merged)

closed via merge request !12891 (merged)

mentioned in commit 1cfa37d6

mentioned in merge request !14785