Standard documentation for software systems

I have been assigned to inventory and create technical documentation for several software systems that have existed for a while. There is some documentation for some of it but I am still trying to determine what’s there. Could you please give your input as to what should be included when the “minimun” and/or “standard” documentation is complete. I am interested in knowing what categories are needed (code libraries, kinds of diagrams, user guides?). Thanks for your help.