I feel like I need a Theory of Documentation

Subtitle: How the heck do I organize this stuff??

I've been part of a 2-person internal IT department for 8 years, and I'm guilty of not really documenting much of anything. There's a bunch of procedural/technical knowledge in my head that doesn't exist elsewhere, and I'd like to fix that.

I'm just starting simple with Onenote. It might be viable to move to a (fairly cheap) dedicated documentation platform, but this is what I have for now.

I started with three major categories:

• Systems ("The Way Things Are.")
• Procedures ("How Do I Do The Thing?")
• Service Providers/Vendors ("Who to Call If...")

I've split Systems out into things like Imaging, Printing, Firewall, AD, Azure, etc. The other two I'm not sure how to split yet, and I think that's where I'm running into problems.

Things that I'm writing under Procedures usually relate to specific Systems. So really, should those things just be lumped together with the Systems? And for that matter, information about our internet provider belongs both under Service Providers and Systems. So... maybe everything is Systems? I keep struggling with the taxonomy / categorization / organization of all this, and it's getting in the way of actually writing the documentation.

So, anyone have a structure I can borrow? Or any thoughts that might help get over this hurdle? Or product suggestions that make this easier?