Turn kinds in python API into a proper Enum (#7686)

Turn kinds in python API into a proper Enum (#7686)

This PR does multiple things:
- the kinds are changed from custom objects to a proper enum.Enum class
  (including according changes to the cython code and the kind generation scripts)
- all examples and tests are modified to account for the change how to use kinds
  (Kind instead of kinds)
- add docstrings to the kind enum values
- add a custom documenter that properly renders enums via autodoc
- extend doxygen setup so that we can write comments as rst (allowing us to copy
  the documentation for kinds from the cpp api to the other apis)