docs(storage): Simplify Python storage example

[brendangadd]

• Add prose to Python self-serve storage example
• Simplify Python code cells
• Create shared "_images" folder
• Update Kubeflow configuration image and move to new folder
• Update all self-storage examples to use updated image

[blairdrummond]

Love the prose. You're right, it benefits a lot from more explanation. I left two inline comments. I think (at the cost of over-duplicating) that it makes sense to leave the image inside the folders for now. (We could add a . to the front of them to hide them in the browser, though?)

[brendangadd]

@blairdrummond Replied to your comments with my reasoning, but this one's all yours. Thanks!