docs: add Ollama + LiteLLM quickstart and sidebar entry (#3176)

env.litellm.example

@@ -0,0 +1,11 @@
+# --- LITELLM (OPENAI-COMPATIBLE) ---
+ENABLE_OPENAI_COMPATIBLE=true
+OPENAI_COMPATIBLE_MODEL_NAME=llama3.1
+OPENAI_COMPATIBLE_API_KEY=sk-test
+OPENAI_COMPATIBLE_API_BASE=http://localhost:4000/v1
+
+# --- DISABLE OTHER PROVIDERS ---
+ENABLE_OPENAI=false
+ENABLE_ANTHROPIC=false
+ENABLE_OLLAMA=false
+ENABLE_GROQ=false