Improve documentation of Bro clusters

Renamed the bro cluster doc to better indicate its purpose (it provides
a high-level overview rather than detailed configuration instructions).
Moved the location of the bro cluster doc in the index so that it makes
more sense (it is an introductory section, not a section about using bro).

Added links in the quick start guide and the bro cluster doc so that
readers can more easily locate more detailed information on configuring
a bro cluster.

Addresses BIT-1160