Restructure the WildFly High Availability Guide

We have realized that the current guide outline does not meet the intended goals and has gotten a bit out of date. Moreover, it's cumbersome to contribute to and fosters nonconceptual contributions.

We (the clustering team) have come up with an outline that addresses the problems, so the first step here is to setup a structure and then push for content - Jiras to follow or we can evolve the existing tracker WFLY-1725.