docs: clarify that image resizing/cropping require sharp to be specified in payload config (#11470)

### 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.

docs/upload/overview.mdx

@@ -185,6 +185,8 @@ The [Admin Panel](../admin/overview) will also automatically display all availab
 
 Behind the scenes, Payload relies on [`sharp`](https://sharp.pixelplumbing.com/api-resize#resize) to perform its image resizing. You can specify additional options for `sharp` to use while resizing your images.
 
+Note that for image resizing to work, `sharp` must be specified in your [Payload Config](../configuration/overview). This is configured by default if you created your Payload project with `create-payload-app`. See `sharp` in [Config Options](../configuration/overview#config-options).
+
 #### Accessing the resized images in hooks
 
 All auto-resized images are exposed to be re-used in hooks and similar via an object that is bound to `req.payloadUploadSizes`.