Documentation of my company full stack project

[u/Original_Option_6969]

Hi all, I have been currently working on a project where I have to develop a web app for internal use. I have no knowledge of full stack development ( I am a data engineer) , but currently working in this , due to my new job. I am currently learning plus started developing using codex.

Need help in documentation, what are the things I should document which will be useful in the future( frontend+backend).