docs: Add more env var examples

.env.example

@@ -1,6 +1,10 @@
 # Example .env file for SlideDeck AI
 # Add your API keys and configuration values here
 
-# OpenRouter API key (if using OpenRouter as a provider)
+PEXEL_API_KEY=your-pexel-key-for-images
 
+TOGETHER_API_KEY=your-together-ai-key
 OPENROUTER_API_KEY=your-openrouter-api-key
+
+RUN_IN_OFFLINE_MODE=true-or-false
+DEFAULT_MODEL_INDEX=3