docs: rewrite skills.json as LLM-friendly self-documenting API reference

Restructured to follow the random.ravelloh.com pattern with:
- Explicit authentication prerequisites and obtain instructions
- typical_workflow guiding agents through the full flow
- Endpoints organized by category (sites, config, analytics)
- Scopes, parameters, and examples per endpoint
- Clear guidance that agents must ask user for API key first

Author: RavelloH