Add reusable card type style guide

[MyouzzZ]

Summary

• Add docs/card-type-style-guide.md documenting Study Copilot card conventions.
• Cover concept, code, quiz, takeaway, and challenge cards with visual/content guidance.
• Include JSON examples, ordering guidance, and a pre-PR checklist.

Closes #22.

Validation

• Documentation-only change; no runtime tests required.
• git diff --check -- docs/card-type-style-guide.md
• Checked card types, accents, and rendering fields against study-copilot-full.html.