docs: add comprehensive Agent Guide for Vapi voice agent configurations

- Introduced a new documentation file, AGENTS.md, detailing the management of Vapi voice agent configurations as code.
- Included quick reference sections for common tasks, project structure, resource formats, and detailed configurations for assistants, voice, models, and transcribers.
- Aimed to provide clear guidance for authors and modifiers of resources without requiring internal knowledge of Vapi.

Author: vtkovapi