Document stability levels

We need docs coverage of the stability levels. Meaning, usage etc.

This should be written in a forward looking way, i.e. describe the generic concepts (the levels) and note that the kinds of things that will vary with level are expected to evolve over time. Plus of course the basic usage stuff, which should already be largely stable.

Also, the differing OOTB levels for standard WildFly vs WildFly Preview should be noted in the WildFly and WildFly Preview document (https://docs.wildfly.org/30/WildFly_and_WildFly_Preview.html).