Format 2 - Allow Reports as second document in YAML

@jxtx really likes the idea of workflow definitions as the "front-matter" to workflow report templates:

An example is here:

https://gist.github.com/jxtx/0525f0a9e1c7f903c21b12f7e29c1b31

The advantages are that it looks a lot like other popular formats (Rmd and Jekyll) and that you aren't embedded Markdown in YAML - which has its problems. My push back would just that it is slightly harder to parse documents that aren't strictly YAML and this would represent a serious divergence from CWL - which we've been trying to converge our workflow format with.

I don't think this is at all incompatible with embedding the workflow invocation report within the YAML so I think we can proceed with merge #8511 without this in place at first. This could probably be implemented 100% in the gxformat2 library after the fact.

Review the linked gist and the gxformat2 library, then compare the proposal with merge #8511. The work is done when the proposed workflow-definition and report-document format is supported without embedding Markdown in YAML, with the parsing behavior documented and verified.

Written by the indexing model from the issue text.