Add "priority" section to Image Optimization doc (#30218)
Add "priority" section to Image Optimization doc (#30218)

This change addresses some feedback that some users don't realize
that the `priority` attribute exists and is critical for managing
LCP when using the next/image component. It adds a dedicated
section to the Image Optimization doc that explains how and when
to use the attribute.

Fixes #29624

## Documentation / Examples

- [x] Make sure the linting passes by running `yarn lint`

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