[docs/example] example on how to interoperate with code expecting pixels as raw data

A common task is to interoperate with code expecting data as raw bytes, eg as a single contiguous array or as an array of padded scanlines.

It would be great to have an example and an entry in the docs.

I'd be happy to make a contribution but the reason I'm asking about it is that I'm not exactly sure how best to do it, short of just copying pixel-by-pixel.

Start by locating the existing examples and documentation for pixel storage and raw image data. Compare the contiguous-array and padded-scanline cases described in the issue, then document and demonstrate the supported interoperability approach; done means both cases have a clear example and documentation entry.

Written by the indexing model from the issue text.