docs: describe EXIF orientation handling

[ishaanlabs-gg]

Summary

• document Jimp's current EXIF orientation behavior in the Getting Started guide
• add the same EXIF orientation note to the package README
• clarify that Jimp does not expose public APIs for arbitrary EXIF tag editing, copying, or preservation when writing images

Fixes #991.

Verification

• git diff --check
• rg -n "EXIF orientation|arbitrary EXIF|getExifOrientation|@jimp/core" packages/docs/src/content/docs/guides/getting-started.mdx packages/jimp/README.md

I did not run the full docs build locally because the external SSD is nearly full and the Jimp workspace dependency install has already failed with ERR_PNPM_ENOSPC in this checkout.