feat: stabilize `unstable_getImgProps()` => `getImageProps()` (#60739)
This PR renames `unstable_getImgProps` to `getImageProps()` (originally
introduced in PR #51205).
Most feedback [after
announcing](https://twitter.com/leeerob/status/1674250190432116736)
looks positive so it seems like we can safely stabilize this API now.
Its unlikely to change.
I also added documentation with example usage.
Closes NEXT-2120
Fixes https://github.com/vercel/next.js/issues/56009