feat: add documentation for public endpoint /api/v1/ai-gateway/providers (#573)

This endpoint is now live and usable. 

<img width="750" height="539" alt="image"
src="https://github.com/user-attachments/assets/9bae99c0-fef6-4156-9057-bb1f52fc5cab"
/>


For data that looks like 
```json
{
  "providers": {
    "anthropic": {
      "token_env_var": "ANTHROPIC_API_KEY",
      "url_env_var": "ANTHROPIC_BASE_URL",
      "models": [
        "claude-3-7-sonnet-20250219",
        "claude-3-haiku-20240307",
        "claude-opus-4-20250514",
        "claude-sonnet-4-20250514",
        "claude-3-5-haiku-20241022"
      ]
    },
    "gemini": {
      "token_env_var": "GEMINI_API_KEY",
      "url_env_var": "GOOGLE_GEMINI_BASE_URL",
      "models": [
        "gemini-2.0-flash-lite",
        "imagen-4.0-generate-001",
        "gemini-1.5-flash",
        "gemini-1.5-flash-8b",
        "gemini-2.5-flash",
        "gemini-2.5-flash-lite",
        "gemini-2.5-pro",
        "veo-3.0-generate-preview",
        "gemini-1.5-pro",
        "gemini-2.0-flash"
      ]
    },
    "openai": {
      "token_env_var": "OPENAI_API_KEY",
      "url_env_var": "OPENAI_BASE_URL",
      "models": [
        "gpt-3.5-turbo",
        "gpt-4o",
        "gpt-4o-mini",
        "gpt-image-1",
        "dall-e-2",
        "dall-e-3",
        "gpt-4-turbo",
        "o1",
        "o1-mini",
        "o3-mini",
        "codex-mini-latest"
      ]
    }
  }
}
```

---------

Co-authored-by: kathmbeck <[email protected]>

Author: rybit