Merge branch 'stable-2.8'

* stable-2.8:
  Correct Gerrit releases download link in installation documentation
  Make the documentation embedding work.
  Update download-commands plugin
  Check preconditions when setting parent project through REST
  Add acceptance tests for setting parent project via REST
  Update release website index to use new documentation site

Change-Id: I0c387d621920913d947ee45da046c9168c0db499