Update plugin development docs

[dsimansk]

A few points to polish around plugin development readme:

• References should point to kn-plugin-sample as base for the plugin
  • (optional) Check and update the sample plugin code if needed
• There should inlined code examples that leverage knative.dev/client-pkg only not client directly
• Double-check if the content is still up-to-date in other parts.

https://github.com/knative/client/tree/main/docs/plugins#kn-plugins

/kind doc
/kind good-first-issue

[JuneOh710]

/assign

[suriananya]

/assign

[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.

[aliok]

/unassign suriananya