Add some simple docs of WildFly Preview.

I'm not sure precisely what but I know I don't want to sprinkle it all over the place. Probably some simple overview of what it is, some discussion of the transformation stuff, a listing of key differences with main WildFly, a caveat on what is less tested. And then some very brief installation sections that note differences from the key points in the docs linked from https://docs.wildfly.org/22/Installation_Guide.html

The Installation Guide itself can probably have a brief bit about WF Preview as well. It's written in terms of making choices about what you want. So perhaps a very brief first bit about choosing regular WF vs Preview, linking to the main WF Preview doc so people can find out more.