content: _index.adoc: add a reference to the CentOS repository
In preparation for the removal of the rhdocs/info directory in c9s && rhel kernels, add a quick link to the CentOS documentation repository.
It may seem obvious to some, but adding redundancy here ensures that users can jump "back" to the documentation source from the rendered adoc pages hosted via Gitlab.
Signed-off-by: Derek Barbosa debarbos@redhat.com