Add User-facing documentation for the Application class

At the moment, only `GUIApplication` is documented in the user-facing docs and `Application` is not. `Application` is fundamental to understanding Envisage Applications so it should be included in the user-facing docs. Note that the following page - https://docs.enthought.com/envisage/envisage_core_documentation/howto_create_a_plugin.html#how-to-create-a-plugin-for-an-envisage-application - might be an okay place to shoehole the `Application` class for the moment.