Added a summary of the whole cartopy package to the docs.

Adds a fairly basic, automated, outline of the cartopy codebase in the documentation. This page will be useful for quickly finding documentation & for developers to identify areas of undocumented code (a lot of it at this stage!).

NOTE: The new page is currently not linked too. There are several such pages and I intend to turn the whole thing into a coherent documentation stream in a separate PR.

reviewer: cpelley