docs: move podman install to its own tutorial

<rant>I attempted to actually run Tutor with Podman and I was sorely disappointed.
The only reliable source of docs that I found concerning the integration with
docker-compose is this blog post:
https://www.redhat.com/sysadmin/podman-docker-compose
There are no other official docs 😓

1. The instructions given in the blog post don't work out of the box. Launching
the podman service failed altogether on Ubuntu 20.04 and 20.10. It worked on
CentOS 8, but some parameters need to changed, such as the docker socket path.
2. After I got the podman service working, I managed to get an Open edX
platform running with tutor, but with the root user. Then, containers
complained that they could not write data to the bind-mounted volumes. I
attempted to run as a non-root user, and discovered that the podman socket is
only readable by root. This should explain why all commands from that blog post
are prefixed by sudo.

Long story short, I was hoping to update the tutorial. Instead, I'm just moving
it for the sake of better organisation. For the life of me, I do not understand
why some people would want to run Podman instead of Docker. Bad documentation
is an immediate turn-off for me. From my perspective, podman is mostly an
overblown marketina stunt.</rant>

docs/install.rst

@@ -107,8 +107,6 @@ After opening a new shell, you can test auto-completion by typing::
 
     tutor <tab><tab>
 
-.. include:: podman.rst
-
 Uninstallation
 --------------
 

docs/tutorials.rst

@@ -17,6 +17,7 @@ System administration
    :maxdepth: 2
 
    tutorials/portainer
+   tutorials/podman
    tutorials/proxy
    tutorials/datamigration
    tutorials/multiplatforms