Add Kudu version number to header of docs pages

It is currently not easy to tell which version of the docs you are looking at when you are on the "unversioned" section of the Kudu docs @ http://kudu.apache.org/docs/ – we should add a header or a little strip to the top of each page that says something like "you are looking at version 1.7.1 of the docs" or "you are looking at docs for version 1.8.0-SNAPSHOT generated from Git commit eee82d90a54108f2d7e18e84ec0bbd391fcc129a"