Turn the Trusted Clusters guide into a tutorial

See: #11841

The Trusted Clusters guide is organized as a conceptual introduction,
with configuration/command snippets used as illustrations. To make this
guide easier to follow, I have structured it as a step-by-step tutorial
where a user should be able to copy each command/config snippet on
their own environment, establish trust between clusters, and connect to
a remote Node.

Some more specific changes:

- Remove Details box re: Node Tunneling: This isn't strictly relevant
  to Trusted Clusters, so removing it shortens and simplifies what is
  quite a long guide.

- Make "How Trusted Clusters work" more concise and add the information
  to the introduction.

- Move long explanatory passages into Details boxes. Eventually, it
  would be great to split this guide into multiple guides that explain
  different topics in more depth (e.g., a section of the docs devoted
  to Trusted Clusters). For now, this is the quickest way to organize
  conceptual information without detracting from the tutorial structure.