Rename abstract_controller/rendering. to errors.rb

Since all rendering stuff was extracted to AV, the only thing that left was single class with error so file name wasn't relevant anymore
Revert "Rename abstract_controller/rendering. to errors.rb"

This reverts commit 6fe91ec5008838338e54ab8570f7c95ee0cdd8e8.
Create AbstractController::Rendering interface

This interface should be use when implementing renderers.
Remove 'api plugin' from docs

AbstractController::Rendering is module which defines thie api