Maybe open a Tracker window on first boot with Quick Tour and User Guide in it? Put a custom background and everything, similar to how macOS software ships their disk images?
Very Haiku-y if you ask me.
How do we edit the documentation? Do we just simply edit the English html file and then everything will carry over to the translated versions?
The QuickTour/UserGuide/Welcome pages are managed by an online tool, see the i18n wiki.
Changes are made in the English language, the translations are as of today handled by the new Google AI service “TransLingua”, see today’s commit. 
There is still a lot of time to consider.
This is Haiku Beta 2 so no need to make big marketing steps now!
Ups maybe wrong thread!?
Anyway the actual User-guide we got is quite good if read by the user!