Make the Labeling guide easier to follow

Backports #12034

* Make the Labeling guide easier to follow

See #11841

The current guide is a conceptual introduction to Teleport's labeling
functionality. Since we cover this information in our reference guides,
I wanted to make it possible for readers to follow this guide as a
tutorial and get started labeling resources quickly.

- Introduce labels at the beginning of the guide to set the stage
  conceptually before the steps begin.
- Clarify in the Prerequisites section that the instructions in the
  guide apply to any Teleport resource, not just Nodes
- Require a host for running a Teleport Node in the Prerequisites and
  include instructions for starting the Node. Previously, we included
  sample commands/config for applications as well. Requiring a Node
  and including setup instructions for that Node will ensure that all
  instructions in the guide can be followed easily.
- Add scoped instructions via Tabs and ScopedBlocks.
- Reorganize sections into a series of steps.
- Add commands to verify that labels have been added.

* Respond to PR feedback

* Improve the Labeling guide for self-hosted users

See: #11841

- Edit tctl.mdx: Users of self-hosted Teleport can run tctl commands
  from their local machines after logging in to a cluster, just like
  they can with Teleport Cloud. I have edited tctl.mdx to instruct self-
  hosted users to use tctl remotely. This way, users won't need to
  access their Auth Service hosts in order to follow the guides that
  use tctl.mdx.

- Add steps for fixing unexpected "tsh ls" output

Also applies PR feedback.

- Add provisions for labeling specific resources as a table near the
  top of the page.
- Remove the option to add labels using CLI flags.
- Fix command output spacing

Also misc. clarity tweaks.

* Respond to PR feedback

docs/pages/includes/tctl.mdx

@@ -4,17 +4,21 @@ scope={["oss", "enterprise"]}
 scopeOnly={true}
 opened={true}
 >
-Verify that your Teleport client is connected:
+
+To connect to Teleport, log in to your cluster using `tsh`, then use `tctl`
+remotely:
 
 ```code
+$ tsh login --proxy=teleport.example.com [email protected]
 $ tctl status
-# Cluster  tele.example.com
+# Cluster  teleport.example.com
 # Version  (=teleport.version=)
-# CA pin   sha256:sha-hash-here
+# CA pin   (=presets.ca_pin=)
 ```
 
-Remain logged in to your Auth Service host so you can run subsequent `tctl`
-commands in this guide.
+You can run subsequent `tctl` commands in this guide on your local machine.
+
+For full privileges, you can also run `tctl` commands on your Auth Service host.
 
 </Details>
 <Details