It's already there! https://guides.getalby.com/user-guide/v/alby-account-and-browser-extension/alby-hub/alby-hub-flavors/umbrel-start9-etc.
Strange. I'm getting a page not found for that link?!
Ah, found it: https://guides.getalby.com/user-guide/v/alby-account-and-browser-extension/alby-hub/alby-hub-flavors/umbrel-start9-etc. - the dot at the end is important but Primal client did use it as a full stop after the link and not as part of the link