### What
Clarifies that `sharp` must be specified in payload config for image
resizing & cropping to work. Also adds link to the configuration page
for further information.
### Why
It is not immediately clear from this single documentation page alone.
While it says that the feature relies on sharp, it does not say that it
must be added to config. Most people won't probably run into this since
they're probably going to use `create-payload-app`, which configures
sharp by default. But those who use custom config (like me) may be left
wondering why this feature does not work.
See [Crop images and preview sizes not
working](https://payloadcms.com/community-help/discord/crop-images-and-preview-sizes-not-working)
in community help.
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