Update how to remove trusted clusters.

[benarent]

Extra docs from #2075 on how to disable trusted clusters.

[webvictim]

This is already in the admin guide: https://gravitational.com/teleport/docs/admin-guide/#disabling-trust

Maybe we just update that rather than having a separate section? Or certainly fix that part too.

[benarent]

I just went ahead and removed / refactored the admin guide and placed most of the contents on the Trusted Cluster page since we had a lot of duplicate content.

[webvictim]

Made a lot of main -> root changes. Also "trusting cluster" becomes "leaf cluster" and "trusted cluster" becomes "root cluster".

We still need to go and edit all these "main" references directly in the trusted cluster file as there are a lot of them. I'll wait for you to apply these corrections, then I can go and clean up the rest on the branch if you want.