Make the three-node minimum more explicit in documentation and on website

One of the most important parts of a production zookeeper deployment is the three-node minimum requirement for fault tolerance ... but when I glance at the website and the documentation, this requirement is difficult to actually find.

It is buried deep in the admin documentation, in a sentence that says "Thus, a deployment that consists of three machines can handle one failure, and a deployment of five machines can handle two failures." Other parts of the documentation hint at it, but nothing that I've seen comes out and explicitly says it.

Ideally, documentation about this requirement would be in a location where it can be easily pinpointed with a targeted URL, so I can point to ZK documentation with a link and clearly tell SolrCloud users that this is a real requirement.

If someone can point me to version control locations where I can check out or clone the docs and the website, I'm happy to attempt a patch.