docs: add API key management guide

[stackbilt-admin]

API key management is now live in the Stackbilder settings page. The docs need a section covering:

• How to generate an API key (Settings > API Keys > Create key)
• Key format: sb_live_ prefix
• Keys are shown once on creation — must be copied immediately
• How to revoke keys
• Using API keys for programmatic access to scaffold and image generation endpoints

This could be a new section in api-reference.md or a standalone page.