Mark assets pipeline guide as "Work in progress" [ci skip]

 - As per rails#19835 (comment)
   assets pipeline guide needs to be rewritten for Sprockets 3.
 - So this commit marks the guide as WIP for now.

guides/source/documents.yaml

@@ -106,6 +106,7 @@
       description: This guide covers the command line tools and rake tasks provided by Rails.
     -
       name: Asset Pipeline
+      work_in_progress: true
       url: asset_pipeline.html
       description: This guide documents the asset pipeline.
     -