Update Screenshot Guidelines

[tomojira]

• Finally updated the list of resolutions
• Added a note on N64 and Saturn screenshot borders
• Added info on 3DS
• Fixed a mistake made about Amstrad CPC
• Added a note on texture filtering for PSP

[greptile-apps]

Greptile Summary

Updates screenshot resolutions and system-specific emulator guidance.

• Adds N64, Saturn, and 3DS guidance.
• Corrects Amstrad CPC cropping guidance.
• Expands accepted dimensions.
• Adds PSP texture-filtering guidance.

Confidence Score: 5/5

The documentation update appears safe to merge.

No blocking or actionable documentation defects remain.

_{Reviews (1): Last reviewed commit: "Update Screenshot Guidelines" | Re-trigger Greptile}