Provide quickstart instructions for using the kubectl workspace plugin

[marun]

Currently there is no obvious documentation of use of the kubectl workspace plugin. The only instructions I could find are bound up on the p2 demo script. Ideally there would be simple introductory instructions for new users in time for p3.

[pweil-]

Last day of P3, moving to P4

[sttts]

This should be based on #744.

[hardys]

/assign @hardys

Discussed with @davidfestal and agreed to take this - AFAICS the current docs are outdated, and need to be updated to reflect the UX implemented via #778 - it'd also be good if the plugin install was linked/mentioned in the top-level readme and developer quickstart docs.

[hardys]

@sttts @davidfestal I have a couple of questions having dug into this:

1. The workspace plurality aliasing seems to be inconsistent, because we only install the ws and workspaces plugins (not workspace) - Do we want to fix this, and if so would another duplicate plugin be preferred, or else perhaps a Makefile target that can create symlinks instead?

$ kubectl get workspaces
NAME    TYPE       PHASE   URL
users   homeroot   Ready   https://10.19.133.66:6443/clusters/root:users
$ kubectl get workspace
NAME    TYPE       PHASE   URL
users   homeroot   Ready   https://10.19.133.66:6443/clusters/root:users
$ kubectl workspaces create foo
Workspace "foo" (type root:organization) created. Waiting for it to be ready...
Workspace "foo" (type root:organization) is ready to use.
$ kubectl workspace create foo2
error: unknown command "workspace" for "kubectl"

2. Somewhat related to the above, the help output is inconsistent - workspace is mentioned as an alias, but also the usage still references kubectl kcp workspace - if we fix the plurality issue, we could detect if/when to include the kcp subcommand prefix, does that sound reasonable?

$ kubectl workspaces help
Manages KCP workspaces

Usage:
  workspace [create|create-context|use|current|<workspace>|..|.|-|~|<root:absolute:workspace>] [flags]
  workspace [command]

Aliases:
  workspace, ws, workspaces

Examples:

	# shows the workspace you are currently using
	kubectl kcp workspace .

	# enter a given workspace (this will change the current-context of your current KUBECONFIG)
	kubectl kcp workspace use my-workspace

[ncdc]

it'd also be good if the plugin install was linked/mentioned in the top-level readme

I am working on this in https://hackmd.io/-YpPqiVsQZyiPIjyoxLiKw

[hardys]

Ok I pushed #1901 aiming to fix the inconsistencies mentioned above, while that doesn't fix this issue it may enable some clearer and/or more consistent docs (I'm assuming we want to switch to referring to the kubectl ws|workspace|workspaces plugin, and eventually deprecate the kcp workspace subcommand?)