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've tried to rephrase the opening to suggest that this is a shortcut for when you have a lot type signatures to declare.  I want this section to be seen as "taking your type signature skills to the next level".  :)

I think the registry is easier to explain once each of the type signature categories have been discussed individually.