Rename introduction.html to index.html and get rid
of old index.html  completely.

Change-Id: Icb4b5b5ba4851d9a7974b413f8637089a157973a
Reviewed-on: http://gerrit.cloudera.org:8080/2675
Tested-by: Kudu Jenkins
Reviewed-by: Todd Lipcon <[email protected]>
(cherry picked from commit 738e99384d4fa3c153922f2debeac3b5772e4c3e)