Document denomination support and improve product update webhook guidance

[nikolayminev-runa]

Description:

Summary

• Added comprehensive documentation for denomination support in product.update webhook events
• Restructured documentation to be customer-focused with actionable guidance
• Updated all examples to match real webhook payload format
• Added detailed usage instructions for preventing order failures

Changes

• New denomination documentation: Fixed vs open types, available_list, min/max values
• Customer-focused guidance: Clear actions for is_orderable, price_multiplier, and denomination changes
• Realistic examples: All JSON examples now match actual webhook payloads with correct price multipliers, timestamps, and complete field structures
• Improved flow: Moved usage guidance before technical reference for better readability
• Added context: Explained purpose of old_state vs new_state comparison

[bhagavansuraj]

lgtm