Manually documenting a database I am forced to manually document a legacy enterprise database. Are there some standard good practices or tools to do this?

To give you some more context, I have recently been given restricted access to a legacy enterprise database. There is very little documentation. I don't have permissions to view system tables or generate ER diagrams. I am exploring/using the database and manually documenting it as I go. 

I would like to represent the schema with code (manually) in such a way that it can be parsed, say, with Python, to generate an ER diagram?

Are there tools to do this kind of thing? Or standard good practices to document the schema, annotate definitions and common keys etc?

I'm imagining some kind of a tree or graph like data structure that can be used to encode the schema, parsed with Python etc, and used to generate ERDs, etc, outside of the db management tool; (since I don't have permissions to do this inside the db management tool (Oracle SQL Developer)).
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?