refactor group docs

- rewrite, restructure index
- move content from `gitlab-basics/create_group.md` to index
- move images to the correct location

doc/gitlab-basics/create-group.md

-# How to create a group in GitLab
 
-Your projects in GitLab can be organized in 2 different ways:
-under your own namespace for single projects, such as `your-name/project-1` or
-under groups.
-
-If you organize your projects under a group, it works like a folder. You can
-manage your group members' permissions and access to the projects.
-
----
-
-To create a group:
-
-1. Expand the left sidebar by clicking the three bars at the upper left corner
-   and then navigate to **Groups**.
-
-    ![Go to groups](img/create_new_group_sidebar.png)
-
-1. Once in your groups dashboard, click on **New group**.
-
-    ![Create new group information](img/create_new_group_info.png)
-
-1. Fill out the needed information:
-
-    1. Set the "Group path" which will be the namespace under which your projects
-       will be hosted (path can contain only letters, digits, underscores, dashes
-       and dots; it cannot start with dashes or end in dot).
-    1. The "Group name" will populate with the path.  Optionally, you can change
-       it.  This is the name that will display in the group views.
-    1. Optionally, you can add a description so that others can briefly understand
-       what this group is about.
-    1. Optionally, choose and avatar for your project.
-    1. Choose the [visibility level](../public_access/public_access.md).
-
-1. Finally, click the **Create group** button.
-
-## Add a new project to a group
-
-There are 2 different ways to add a new project to a group:
-
-- Select a group and then click on the **New project** button.
-
-    ![New project](img/create_new_project_from_group.png)
-
-    You can then continue on [creating a project](create-project.md).
-
-- While you are [creating a project](create-project.md), select a group namespace
-  you've already created from the dropdown menu.
-
-    ![Select group](img/select_group_dropdown.png)
+This document was moved to [another location](../user/group/index.md#create-a-new-group).