Define the namespace / organization / group / subgroup / project object model
See originating comment: https://gitlab.com/gitlab-org/gitlab-services/design.gitlab.com/-/issues/908#note_575210186
## Issue
Properly add documentation on the logical object model for:
- _Organization_
- `Group` (`NAMESPACE`)
- `Project` (`NAMESPACE`)
- `Subgroup` (`NAMESPACE`)
- `Project` (`NAMESPACE`)
Also add examples.
| Complex | Simple |
| ------ | ------ |
|  |  |
## For reference
**https://gitlab.com/groups/gitlab-org/-/epics/4257#note_401529384**
issue