Pre-RFC: doc-dependencies

This roughly agrees with my thoughts in Explicit "test" dependency grade? that the existing dependency grades we have are far too generic to be useful for various tasks, especially that they burden working on other targets with the need to compile the dependencies due to some unrelated target being forced to use the same dependency grade.

That we already have build-dependencies is a step in the right direction.

( deps required only for examples are a particular fly in my ointment )