Add subgroups limitations to Pages docs

See the general Documentation guidelines http://docs.gitlab.com/ce/development/doc_styleguide.html

What does this MR do?

Pages doesn't work with subgroups. We added this limitation to the Subsgroup doc, but not to Pages. This MR fill this gap.

EE-version: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/2444

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34621

approved this merge request

changed milestone to %9.5

added Documentation label

added pages label

merged

mentioned in commit 1cfa37d6