Add missing empty states in Admin Area
### Description
Within the instance-level GitLab Admin Area, there are a lot of configuration options. Most of these are optional and naturally default to "not set up".
While few of these have a clear information structure and an engaging empty state ([ConDev Index](https://gitlab.com/gitlab-org/gitlab-ce/issues/30469)!), most of these pages lack UI polish today.
### Proposal
Let's analyse and improve all pages and configurations that miss a user-friendly empty state within Admin Area, as a first step.
Please be aware that some of these empty states are not covering the full content page, but are part of additional page sections.
I'd love to encourage and see illustration ideas when we find time!
epic