[Documentation] Add developer documentation for safetycases

• Overview in html/developer/documentation/documentation.html. See e.g. af3.state, af3.timing, af3.allocation for examples
• Documentation of metamodels (at least all metaclasses have to be documented)

(from redmine: issue id 3753, created on 2019-06-21)

• Relations:
  • parent #3396 (closed)