chore: unpublish /inference/chat-completion (#3609)

# What does this PR do?

BREAKING CHANGE: removes /inference/chat-completion route and updates
relevant documentation

## Test Plan

🤷

docs/docs/building_applications/playground.mdx

@@ -44,7 +44,7 @@ The playground provides interactive pages for users to explore Llama Stack API c
 
 **Simple Chat Interface**
 - Chat directly with Llama models through an intuitive interface
-- Uses the `/inference/chat-completion` streaming API under the hood
+- Uses the `/chat/completions` streaming API under the hood
 - Real-time message streaming for responsive interactions
 - Perfect for testing model capabilities and prompt engineering
 

docs/docs/building_applications/telemetry.mdx

@@ -313,7 +313,7 @@ client = LlamaStackClient(
 )
 
 # All API calls will be automatically traced
-response = client.inference.chat_completion(
+response = client.chat.completions.create(
     model="meta-llama/Llama-3.2-3B-Instruct",
     messages=[{"role": "user", "content": "Hello!"}]
 )
@@ -327,7 +327,7 @@ with tracer.start_as_current_span("custom_operation") as span:
     span.set_attribute("user_id", "user123")
     span.set_attribute("operation_type", "chat_completion")
 
-    response = client.inference.chat_completion(
+    response = client.chat.completions.create(
         model="meta-llama/Llama-3.2-3B-Instruct",
         messages=[{"role": "user", "content": "Hello!"}]
     )