Document initialization procedure

[dssgabriel]

The documentation should include a section detailing how to initialize KokkosComm applications, with robust code snippets that users can easily copy-paste.

[cedricchevalier19]

I would rather have functions that the user can call.

We can be open-minded and not make them mandatory but I think providing examples to copy and paste is a recipe for disaster. We can change the internals of our functions we will be stuck forever if we do not want to break user code.