Cleanup filetree and improve documentation

In this MR I add descriptions to elements used in the process of creating flatpak images, and rework the elements/ directory to be clearer.

There are 2 changes I make to the elements/ directory:

1. Move the actual flatpak images into a separate subfolder

   • This will make it clearer which elements are our actual end products, and also mitigate confusing filenames.
   • There is a stack element in the elements/flatpak-images/ directory - platform-vaapi.bst, but given this is what we actually build in all.bst, and that it's just there to avoid building the vaapi extension on arm and functions as a flatpak_image element I think this makes sense.

2. Move the extensions into a separate subfolder

   • This makes it easier to find where extension-specific data is held
   • Additionally makes our elements/ folder less of a mess

I also remove the redundant base-sdk production.

Part of #283 (closed)