Move core examples/best practices to appendixes

These sections are not normative (RFC 2119 terms will be
removed in a subsequent commit), and moving them to an
appendix reduces the length of the sizable normative
portion of the spec.  And also improves flow, I think.

The examples within core tend to rely on understanding more
of the core keywords than have necessarily been introduced,
so they work better after the entire vocabulary has been
introduced.

Author: handrews