Manually documenting a database
Since asking the question, I have found these two very nice web-based solutions:
https://dbdiagram.io/home
https://alternativeto.net/software/quickdbd/
and a basic one:
https://dshifflet.com/erd/index.html  

Unfortunately due to security concerns, I am not allowed to use 3rd party cloud solutions. It's gotta be a standalone desktop software. 

I also discovered ERAlchemy and it looks like I can manually generate ERDiagrams in Typora using markdown. 

Any other useful ideas?