There is an integrated diagram editor, which uses a fork of the open-source draw.io.
Press ^⇧D to insert a diagram into a page or double-click an existing diagram. This will open a new window with the diagram editor.
The diagram will be synced to the notebook after each edit.
⚠️ Make sure that you don't close the browser window of the notebook, otherwise the diagram data can't be saved to the notebook.