docs: use an Excalidraw image for Storage model

[Lenormju]

Suggestion : use Excalidraw for schemas Actually : there is only one schema that is relevant to be made in Excalidraw, so not sure if it is relevant.

Facts about sizes :

• without the Excalidraw metadata, the image is 30.2 Ko
• with the Excalidraw metadata, the image is 33.9 Ko
• the original image is 47.1 Ko