Define the namespace / organization / group / subgroup / project object model

See originating comment: #908 (comment 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

gitlab-org&4257 (comment 401529384)