Create user/developer documentation

[fabianoboril]

We should create proper documentation with ReadTheDocs for ScenarioRunner.

User documentation should cover:

• [ ] Documentation for python-written scenarios
  • [ ] Documentation for all atomics that can be used
  • [ ] Documentation for the BasicScenario class
  • [ ] Example
• [ ] OpenSCENARIO scenarios
  • [ ] Documentation of OpenSCENARIO
  • [ ] List of supported/unsupported features
  • [ ] Example
• [ ] Usage with ScenarioRunner

Developer documentation should additionally cover:

• [ ] Documentation of ScenarioManager
• [ ] Documentation of CarlaDataProvider
• [ ] Documentation of ScenarioRunner class
• [ ] Documentation of ResultWriter

The API documentation can be done with mkdocstrings (https://github.com/pawamoy/mkdocstrings) Maybe we need to use Python mocking to work around missing imports on Travis (e.g. numpy, carla, etc.)

[werewolfdev]

@fabianoboril Thanks a lot for this .Looking forward to this docu