Even when object storage is enabled, Pages currently requires
the NFS mount to be available for the deploy tasks to work.

This change adds a note to the documentation instructing that
the NFS mount not be removed when attempting to configure
object storage for pages.

Related epics:
- https://gitlab.com/groups/gitlab-org/-/epics/3910
- https://gitlab.com/groups/gitlab-org/-/epics/3901