Document the kn CLI on the site

[dwelsch-esi]

Overview

The kn CLI documentation is maintained in the
[/knative/kn] repo, except for the install instructions, which are on the doc
site.

Consolidate the kn CLI documentation so that it's maintained in one place.

Audience: Admin and developer

Type: Reference

Context

This issue tracks recommended changes resulting from an analysis of the Knative
documentation commissioned by CNCF. The analysis and supporting documents are
here: https://github.com/cncf/techdocs/tree/main/analyses under 0015-knative.

Possible Implementation

Related material in the current doc:

• https://github.com/knative/

Suggested changes:

Document the kn CLI on the documentation website rather than linking to the
[/knative/kn] repo documentation.

[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.

[skmahe1077]

/assign