Here's the first of a few templates I've gathered or made throughout the years. The Company Infrastructure template.
This "catch-all" document can be a central repository for network, system and vendor information, something to use to survey the environment for an audit, or when handing off to / indoctrinating a new IT person. Not all of the entries may be relevant and there will be some line items that need to be added, but hopefully this help you get started.
Here's an abridged overview I created for a client, much of it has been gutted and fictious information added to obfuscate the company.
The template is in PDF format, but if there's a request for something editable, I will make the Word format available as well.