[doc] minor restructuring

doc/MODULE_MIGRATION.md

@@ -1,10 +0,0 @@
-# Migration of modules
-
-- bumblebee.util.which has been replaced with shutil.which
-- bumblebee.engine.Module has been replaced with core.module.Module
-- module __init__ has less parameters
-- super() works differently
-- engine.input.register_callback is now core.input.register
-- update() only has a single parameter (self) (no widgets anymore)
-- bumblebee.util.format stuff moved into util.format (byte, aslist, asbool, etc.)
-- util.format.duration -> shorten -> compact, suffix -> unit