Improve `knb` test coverage and docs

[dsimansk]

This is follow-up to initial work done in the PR https://github.com/knative/client/pull/1226 to add knb tool to enhance plugin inline support.

TODO:

• [ ] Generate commands documentation in similar way as kn
• [ ] Specific plugin examples with plugins + yaml configs
• [ ] Add test coverage with unit & E2E tests

[github-actions[bot]]

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