Document the kn plugin directory

[rhuss]

• The kn plugin directory is ~/.config/kn/plugins on Linux and macOS, and on %APPDATA%\kn\plugins (I think this information is also missing on https://knative.dev/docs/client/kn-plugins/#manually-install-a-plugin where it is only mentioned that it can be put on the $PATH

Originally posted by @rhuss in #5167 (comment)

[abrennan89]

The plugin directory config and how to use it is documented here: https://knative.dev/docs/client/configure-kn/