feat: add Together AI integration and provider implementation guide

- Create detailed provider implementation guide with:
  - Architecture overview and implementation steps
  - Configuration patterns and best practices
  - Testing checklist and Docker integration guide
  - Example using Together AI implementation
- Add Together AI as new provider with:
  - Environment variables and Docker configuration
  - Support for Qwen, Llama, and Mixtral models
  - API key and base URL management
  - OpenAI-compatible API integration

app/lib/.server/llm/model.ts

@@ -146,6 +146,8 @@ export function getModel(provider: string, model: string, env: Env, apiKeys?: Re
       return getGoogleModel(apiKey, model);
     case 'OpenAILike':
       return getOpenAILikeModel(baseURL, apiKey, model);
+    case 'Together':
+      return getOpenAILikeModel(baseURL, apiKey, model);
     case 'Deepseek':
       return getDeepseekModel(apiKey, model);
     case 'Mistral':