Summary
rhn-support-zanini provided this doc [1] for the reference. We need to include something similar in midstream https://github.com/openshift-knative/docs

[1] https://gist.github.com/ricardozanini/2129fa6b07deff7497221f72d73520ac

Goal
Update the kn workflow cli chapter to include the `kn workflow gen-manifests` command.

Acceptance criteria
The kn workflow cli guides reference `gen-manifests` and provide simple procedure about the command usage.