Keeping client sites maintainable when each uses a different stack

I inherited responsibility for a handful of small client sites. One is plain PHP, another WordPress, another a JavaScript build that needs several packages updated before it will run locally. None is huge, but remembering their individual deployment quirks is getting ridiculous. I'm considering a one-page maintenance note per site: runtime version, repository, deploy steps, backups and known fragile bits. What else would you document first so the next person isn't reverse-engineering the project from scratch?