Add a 'targets.yaml' files in /specification
We're well beyond the point now where a simple `cat` of .rst files to "build"
the spec is practical. We may want to slice and dice the spec in different
ways to address various cross-cutting concerns. To this end, there is now a
'targets' file which contains the "build targets" for the spec, which contains
the sorting order for the .rst files. For now, we just have a single
target: 'main'.
