Updated routing.rst to clarify and expand it

Split "Routing Definition" into two sub-sections and clarified what baseRouteName does and how all the action routes' names are generated

Listed the default routes defined by Sonata

Added note about how to see the current routes for an Admin (via the console)

Added note that removing a route stops Sonata from linking to that action

Added sub-headings to the "Removing a route" section

Cleaned up the "Default route in List Action" section

Tidied up and expanded the "Create a route" section