Documentation III

[jhugman]

Fixes #121

This PR completes the baseline documentation (I think).

It adds:

• internals - sketching:
  • the codegen'd path that the generate turbo-modules uses
  • the lifting and lowering path that the generated bindings uses
• callback interfaces and async-callbacks
• contribution guide for documentation.
• miscellaneous typo fixes and refactoring

Reviewing theses docs has a guide now!

@Johennes : this page could do with your review/input.