Documentation: setup: Add notes regarding podman and SELinux

The setup instructions are wrong for (rootless) podman.
Add notes helping with getting paperless-ngx setup with podman.
For rootless podman the process inside the container should be running as root (uid 0), which corresponds to the user running the container.
The setup as described may lead to SELinux preventing access to consume and export which prevents the container from accessing those.

docs/setup.md

@@ -98,6 +98,10 @@ steps described in [Docker setup](#docker_hub) automatically.
     Don't change the part after the colon or paperless wont find your
     documents.
 
+    !!! note
+
+        When running with SELinux, this may need be taken into account e.g. by adding :Z for consume and export.
+
     You may also need to change the default port that the webserver will
     use from the default (8000):
 
@@ -137,6 +141,10 @@ steps described in [Docker setup](#docker_hub) automatically.
     >   user: <user_id>
     > ```
 
+    !!! note
+
+        This is not required for podman and will break things. Similarly `USERMAP_UID` and `USERMAP_GID` should be unset or set to 0.
+
 5.  Modify `docker-compose.env`, following the comments in the file. The
     most important change is to set `USERMAP_UID` and `USERMAP_GID` to
     the uid and gid of your user on the host system. Use `id -u` and