Structure and extend help message

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 --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.

No file or test is named. Start by locating the Go CLI entry point that defines the help message and compare its output with the structure shown in the issue. Done means the commands are grouped into sections and useful command examples are included in the help output.

Written by the indexing model from the issue text.