docs: Add explanation of imgid vs id for issue #50

[manyworldss]

Description

Added documentation explaining the difference between imgid and id for product images.

This PR:

• Explains what each identifier represents
• Shows how they relate to each other
• Provides examples of URL structures
• Shows an example use case
• Includes a sample API response

Related Issue

Closes #50

Additional Context

This is my first contribution to Open Food Facts. I'd appreciate any feedback!