Change --dryRun semantics and add dedicated --resolve

[g-pavlov]

What this PR does / why we need it:
The current dryRun shows only the resolve documentation structure, but it does not reveal details about how it will be serialized.
This PR changes the behavior as follows.

• dry-run flag
  A command run with dry-run option will now use the writers.DryRunWriter provided for that purpose both for node and
  resource writing. The writer implements the writers.Writer printing the resulting file/folder hierarchy to stdout, plus statistics
  about the files processing (e.g. links):

  dev
    doc
      aws_provider.md
        Links stats: 0 link rewrites
      gardener
        README.md
          Links stats: 85 link rewrites
            ./concepts/apiserver -> concepts/apiserver.md
            ./concepts/apiserver_admission_plugins -> concepts/apiserver_admission_plugins.md
            ./concepts/controller-manager -> concepts/controller-manager.md
  ...
  Build finished in 2.432423 seconds
  

• resolve flag
  The resolve flag prints out the resolved documentations structure, expanding nodeSelector nodes to node hierarchies and removing the selectors afterwards. When resolve is used with nodeSelector it is useful for quickly scafolding structures from existing file hierarchies.

Release note:

Introduced a new `--resolve` flag. It prints to the standard output the resolved documentation structure, expanding `nodeSelector` elements to node hierarchies. When `resolve` is used with `nodeSelector` it is useful for quickly scafolding structures from existing file hierarchies.

Changed the `--dry-run` flag behavior. When running docforge with `--dry-run` it will do a full scale build as instructed, but without writing down the result. Instead, it will print to the standard output the projected file/folder structure with statistics for the processing of each document in the structure and total processing time.