Add deprecation warning for govuk_assets_path

Setting govuk_assets_path should no longer be necessary, and to keep the
code simple it will no longer have any effect. This commit lets users know
about this change in case it causes them some surprise.

lib/govuk_tech_docs.rb

@@ -24,12 +24,24 @@
 require "govuk_tech_docs/api_reference/api_reference_extension"
 
 module GovukTechDocs
+  class GovukTechDocsExtension < Middleman::Extension
+    def after_configuration
+      if app.config[:govuk_assets_path]
+        warn "Warning: setting govuk_assets_path in config.rb is no longer necessary, and will have no effect."
+      end
+    end
+  end
+
+  ::Middleman::Extensions.register(:govuk_tech_docs, GovukTechDocsExtension)
+
   # Configure the tech docs template
   #
   # @param options [Hash]
   # @option options [Hash] livereload Options to pass to the `livereload`
   #   extension. Hash with symbols as keys.
   def self.configure(context, options = {})
+    context.activate :govuk_tech_docs
+
     context.activate :sprockets
 
     context.sprockets.append_path File.join(__dir__, "../node_modules/govuk-frontend/")