Structure and extend help message

[rhuss]

Move some not so important message into an own section, like for kn:

kn is the command line interface for managing Knative Serving and Eventing resources

 Find more information about Knative at: https://knative.dev

Serving Commands:
  service      Manage Knative services
  revision     Manage service revisions
  route        List and describe service routes
  domain       Manage domain mappings
  container    Manage service's containers (experimental)

Eventing Commands:
  source       Manage event sources
  broker       Manage message brokers
  trigger      Manage event triggers
  channel      Manage event channels
  subscription Manage event subscriptions

Other Commands:
  plugin       Manage kn plugins
  completion   Output shell completion code
  version      Show the version of this client

Use "kn <command> --help" for more information about a given command.
Use "kn options" for a list of global command-line options (applies to all commands).

Currently, the help message looks a bit unstructured.

Also, adding examples would be very helpful as the help message will probably be the only documentation for the plugin for some time.

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Reopen the issue with /reopen. Mark the issue as
fresh by adding the comment /remove-lifecycle stale.

[rhuss]

/remove-lifecycle stale