In these arrays you can give on array of fields for translatable fields (create & edit) and non translatable fields (create & edit). Those will be shown on the page create and edit views.
Defining the relation
In that same config file you'll also see a relations key, just as with the User module, you can add dynamic relations with the Page entity. This will mostly be with your custom module that will have the entity with the additional data to be stored
The way to 'intercept' the posted data, is by hooking into the PageWasCreated and PageWasUpdated events. These events both have the pageId (int) and data (array). The data contains the posted data.
With the configuration setup, and your event handlers in place, you'll have the custom data stored in database. To display the data you can simply call the relation you setup in the configuration as a method.