Link code snippets from existing examples in KUDO

[nfnt]

What would you like to be added:
Currently, the example YAMLs in, e.g., Developing Operators are part of the documentation. To always ensure their correctness, they should be code snippets taken from the examples provided by KUDO instead. This could be done by adding a pinned version of KUDO as a Git submodule to this repository.

Why is this needed:
This will ensure that the examples in the documentation work. Especially once their correctness is ensured with integration tests (e.g., kudobuilder/kudo#828).