[Docs] Many docs improvements

Introduce Backends under Concepts with backend configuration examples moved from the reference
Improve the reference structure by making it easier to read

Improve the reference structure by moving it before examples
Move Projects from Concepts to Guides and rename it to Administration (to focus not only on projects but on all aspects of administration)
Ensure Dev environments, Tasks, and Services are also under /concepts
Move major examples from the reference page to Concepts (e.g. Distributed tasks, Ports, Replicas and scaling, etc.)

Navigation Menu