Remove compatibility module from docs [ci skip]

This module exists to warn old users. I think we should remove
it from the docs so we don't advertise it.

actionpack/lib/action_controller/template_assertions.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 module ActionController
-  module TemplateAssertions
+  module TemplateAssertions # :nodoc:
     def assert_template(options = {}, message = nil)
       raise NoMethodError,
         "assert_template has been extracted to a gem. To continue using it,