docs: invite people to define GPT partition types for all archs now

docs: invite people to define GPT partition types for all archs now

The discovery partitions spec so far suggested we should define
arch-specific partition type uuids only for archs that have EFI. Let's
change that and invite people to define them for any arch. Why? Even if
GPT is defined as part of the UEFI spec it's quite useful independently
of it, too. Specifically, our image dissection logic makes use of it,
i.e. systemd-nspawn, systemd-gpt-auto-generator, systemd-repart,
RootImage=, portable services, and so on. None of these tools are
related to UEFI in any way.

Hence, let's open this up.