Create separate generation phase for docs

[petertrr]

I'm starting to think that we need a separate file for such tasks with documentation in order not to run them during every build, maybe in a separate maven profile. But it can be done later in a separate PR.

Originally posted by @petertrr in #586 (comment)

[petertrr]

The fundamental problem here is that we have code/docs generation in main source set, and these classes are included in the jar. We can exclude dependencies when adding diktat-rules in plugins' poms, but more correct solution would be to extract codegen utils to a separate maven module (opened #592 or this).

originally posted: #584 (comment)