what you use for your documentation
Hugo (markdown) files that i host on my internal server.
how you organize it
I use basic directory structure. Top level directories are like "dev", "home", "general". Self hosting is a dev/ subdir.
what information you include
Depends on how familiar i am with it and how often I'll be referencing it. Something i know well or access often will be more high level. Things like an annual process i have documented in more detail
how you work documentation into your changes/tinkering flow
My site has an "edit this page" feature which i use to open my IDE and make the change as I'm doing things. Sometimes I'll be lazy and just add in what i did this time and then let future me reconcile the differences 🙃