Add comprehensive documentation for custom API endpoint configuration
to address missing documentation reported in issue #567.
Changes:
- Create docs/custom-providers.md with detailed guide for custom: and anthropic-custom: formats
- Add custom endpoint examples to README.md configuration section
- Add note about daemon requirement for channels in Quick Start
- Add reference link to custom providers guide
Addresses: #567
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
