Ubuntu Installation Guide (18.04) contribution

[Frank Heimes]

Hello - please has someone an idea on how I can contribute to the "Ubuntu
Installation guide(s)"?
https://help.ubuntu.com/16.04/installation-guide/

I especially want to contribute to the "s390x (IBM System z)" version of
the Installation Guide.
[Well, even the name is nowadays wrong and should be changed to "s390x (IBM
Z and LinuxONE)"]

I found that link to the DocTeam with all it's information:
https://wiki.ubuntu.com/DocumentationTeam
but looks like the focus there is mainly Desktop, Server Guide and Ubuntu
Manuals - am I wrong?

I know howto contribute to the Server Guide, but it looks the it's
different for the Installation Guide.

There are btw. lot's of things I would like to change - I copied and
modified the 16.04 version already and put it temporarily into this wiki:
https://wiki.ubuntu.com/S390X/InstallationGuide
But for 18.04 I would love to have all needed modifications and addons in
the official guide ...

Any idea is appreciated, thx

Frank