Document AI prompt structure for study card generation

[MyouzzZ]

Summary

• document the topic, notes, and PDF prompt entry points used for AI study card generation
• explain the supported concept, quiz, takeaway, challenge, and code card types
• capture output schema expectations, fallback behavior, and prompt improvement ideas

Closes #12.

Validation

• Reviewed generate_cards_with_ai, generate_cards_from_notes, and fallback card generation in backend/main.py
• Reviewed frontend card type rendering in study-copilot-full.html
• git diff --check -- docs/ai-prompt-structure.md