[img-conversion] Examples from spec should be in explainer (#8)

The explainer says:

> This includes use cases where you simply want to convert between types, such as getting from `Blob` to `ImageData` which requires use of an intermediate canvas (example 1 in the spec).

We should move that out of the spec into the explainer. The spec should not concern itself with the world as a it is today, only with the world as it will be once the spec is implemented. 

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/8