API projects documentation grouped into sections & updated with status codes

The API documentation for projects now is structured into major sections that describe
the different aspects when dealing with projects, e.g. hooks, branches, team members etc.
All described methods now contain a list of possible status codes the method can return. A few
methods have extra sample JSON responses and a description if a method is idempotent.
Idempotent methods can be called multiple times while returning the same status code.