This is one of those "It depends..." questions, where it really does - on so many variables that it would be a major chore just to list them all. So an easier way to go about just establishing an idea of where to put the first bite would be to get an idea of the lateral limits. You are already familiar with the startup side of the spectrum, so spend a little time thinking about the extreme opposite: multi-century continuity, the US military's system of knowledge transfer. To be clear - you really don't want to emulate it, even to the degree that IBM did with their manual that helpfully informed employees of the correct way to sit at a desk, but know what the extreme looks like. Maybe pick a few high level concepts out of it:

* Graduated levels of mastery: few people need the details for standing up a production database, but all department members need to know that a company process already exists for it.

* Training dependencies: the mapping doesn't have to be perfect, a little goes a long way in giving shape to a training program.

* Training records: log employee training sessions, from self declarations of 'Yes I read this brief' to consultant led seminars.

Basically 80% of the military's comprehensive training program can be implemented with a network drive of docs and a spreadsheet. A couple of places I contracted for was pretty heavily reliant on Microsoft Sharepoint. I'd be surprised if there wasn't a fairly decent open source suite of components that could give you a solid starting point for a training program what would be somewhere in the middle of the spectrum.