* Update podman quadlet docs to include companion
The prior documentation appears to predate the introduction of the
companion container. This commit adapts the instructions to include the
new companion and some improved formatting.
* fix admonition
* correct contributor name
Some of lzap's original wording remains, but the guidance has been
rewritten from scratch. Perhaps their username should remain for some
time longer, but they noted that they no longer use Invidious #602 also.
* comment out conflicting env var
* set SELinux label for volumes
`:Z` means the volume is not shared between containers. `:z` allows
other containers to share the volume, implying read/write.
Most volumes should be labeled `:Z` unless the volume must be shared
between multiple containers.
