Add documentation surrounding docker-in-docker.

[peter-mcclonski]

For users not intimately familiar with how Rancher Desktop functions, it can be confusing to set up docker-in-docker when Rancher Desktop is not running in admin mode. Most existing references on the wider internet recommend volume-mounting the docker socket from the host system into the DinD container. To a naive user running Rancher Desktop without admin rights, this would imply mounting $HOME/.rd/docker.sock (host) to /var/run/docker.sock (container). Unfortunately, this approach is wrong-- Users must understand the architecture of Rancher Desktop to understand that they must volume mount /var/run/docker.sock, which to them would naively appear to be a nonexistent file.

I propose addition of a Docker In Docker how-to page, or at minimum, a note on an existing page, explaining how to mount the host socket, and a very high level explanation of the 'why' behind it. This would improve approachability for non-experts seeking to use DinD workflows.