How do you document what you are learning at work?

We run a whole bunch of micro-services at work and I keep discovering things about them when I am reading through the code, fixing bugs, runbooks or whatever else. Right now, I have a note for every service and I keep appending to that. Evernote has some serious issues - no versioning, easy to totally delete the note (no warning). How do you all keep track of all the incremental learning?