Make the OpenSSH guide more usable

See #11841

- Add a Prerequisites section and divide the instructions into steps via
  H2 headings.
- Use ScopedBlocks to add instructions for Cloud users.
- Fill in missing instructions (e.g., adding host_cert permissions for
  Cloud users and editing the file permission of the host key/cert)
- Since all supported versions of Teleport now include "tsh config,"
  remove the manual OpenSSH configuration option. This makes the guide
  much simpler to follow.
- Add a Details box explaining "tsh config" The original OpenSSH guide
  included some technical details about the proxy subsystem, but
  without enough context to understand the configuration generated by
  tsh config. I've moved the technical details into a Details box, and
  expanded them, so readers interested in how the local SSH config they
  generate via "tsh config" works.
- Indicate that you need to specify the SSH port when connecting to an
  OpenSSH host via configuration generated by tsh config.

docs/pages/architecture/tls-routing.mdx

@@ -85,8 +85,8 @@ which can be used as a `ProxyCommand`.
 Similarly to `tsh ssh`, `tsh proxy ssh` establishes a TLS tunnel to Teleport
 proxy with `teleport-ssh-proxy` ALPN protocol, which `ssh` then connects over.
 
-See [OpenSSH client](../server-access/guides/openssh.mdx#use-the-openssh-client-to-access-teleport-nodes)
-guide for details on how it's configured.
+See [OpenSSH client](../server-access/guides/openssh.mdx) guide for details on
+how it's configured.
 
 ## Reverse tunnels