Add a how do I? page

[dstansby]

This should be a nice complement to the tutorial, and be a place to collect 'user stories'. I've only added one example at the moment, but we should certainly add more (contributions welcome!)

[will-moore]

Feel free to include the example I had at #121 (comment).
Just needs the exclude_none tweak you mentioned:

for k, v in image.model_dump(exclude_none=True).items():
    root.attrs[k] = v

[dstansby]

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