[RFE] Documentation around available sysext must be clearer

[tormath1]

Current situation

Flatcar provides three types of systemd-sysext images:

• non-official: from the Flatcar sysext-bakery releases
• official: like ZFS, Podman or Python (one needs to write the extension name in /etc/flatcar/enabled-sysext.conf)
• built-in: like Docker, Containerd, OEM partition

But the difference is not very clear from a user point of view.

Impact

It's confusing to know which sysext is available / built-in / etc. For example there is a Docker image from the bakery but there is also a Docker built-in sysext image. (read: https://github.com/flatcar/Flatcar/issues/1470 or https://github.com/flatcar/Flatcar/issues/1481)