What's new in Gerrit 2.8: More info about plugins

- Features are also demonstrated in the core plugins

- Clarify that plugin provided download schemes are for changes;
  project download commands are still provided by core Gerrit.

- Core download schemes and commands are moved to the new
  download_commands core plugin.

- The Cookbook Plugin replaces the Hello World plugin.

Change-Id: Ic564994d9781c5a99ee650028c71eec94e2ed30e