[img-conversion] Explainer should explain how to use the API for common cases (#11)

The explainer gives great justification as to why the API is needed + use cases, but lacks a section on how to actually use the API in common cases. Having the common cases of use is important as it gives people a better sense of how they can use the API in practice. 

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Reply to this email directly or view it on GitHub:
https://github.com/WICG/img-conversion/issues/11