Improve the definition of 'group's dashboard' and update screenshots
We use the term group's dashboard to refer to the page where users manage their existing groups and create new ones. This is somewhat confusing for admin users who may assume that group's dashboard can be found at
/admin/groups/<group-name> - a logical assumption as this page has the feel of a dashboard (e.g. manage group and users).
We could remove any ambiguity by explicitly stating when an action is to be carried out in the admin panel vs. a normal user's interface.
In addition, several of the screenshots for adding and managing groups are outdated.
Mentions of group's dashboard in docs:
- https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/workflow/share_projects_with_other_groups.md (contains outdated screenshots)
- https://gitlab.com/gitlab-org/gitlab-ee/blob/master/doc/workflow/groups.md (contains outdated screenshots)