DevDocs: Extending the Front-end

Some documentation on how to perform common tasks, like creating a new Numba type and registering type signatures for functions and attributes.

I think using the word "intrinsic" here only obscures the message. There is nothing terribly special about ``a + b`` compared to a function call (after all, in regular Python it *is* a method call :-)). So perhaps you can talk about "operations" and mention that they include operators and function / method calls ; you may also mention that it can be explicit or implicit (an example of implicit operation is when calling iter() on the iterable argument of a for loop).