Improvements to melange YAML reference

[erikaheidi]

The melange YAML is a bit more complex (as in, has more nodes and levels) than the apko YAML,so the template I used with apko may not work that well for melange. This will require a few more nested sections, maybe a second level reference table.

Capturing here requests from @amouat on #162 :

• we need an explanation for each of the built-in pipelines
• some fields have subfields that aren't described e.g. copyright