chore: use uvicorn to start llama stack server everywhere (#3625)

# What does this PR do?
https://github.com/llamastack/llama-stack/pull/3462 allows using uvicorn
to start llama stack server which supports spawning multiple workers.

This PR enables us to launch >1 workers from `llama stack run` (will add
the parameter in a follow-up PR, keeping this PR on simplifying) by
removing the old way of launching stack server and consolidates
launching via uvicorn.run only.


## Test Plan
ran `llama stack run starter`
CI

docs/docs/concepts/apis/external.mdx

@@ -357,7 +357,7 @@ server:
 8. Run the server:
 
 ```bash
-python -m llama_stack.core.server.server --yaml-config ~/.llama/run-byoa.yaml
+llama stack run ~/.llama/run-byoa.yaml
 ```
 
 9. Test the API:

docs/docs/deploying/kubernetes_deployment.mdx

@@ -170,7 +170,7 @@ spec:
       - name: llama-stack
         image: localhost/llama-stack-run-k8s:latest
         imagePullPolicy: IfNotPresent
-        command: ["python", "-m", "llama_stack.core.server.server", "--config", "/app/config.yaml"]
+        command: ["llama", "stack", "run", "/app/config.yaml"]
         ports:
           - containerPort: 5000
         volumeMounts:

docs/docs/distributions/k8s/stack-k8s.yaml.template

@@ -52,7 +52,7 @@ spec:
           value: "${SAFETY_MODEL}"
         - name: TAVILY_SEARCH_API_KEY
           value: "${TAVILY_SEARCH_API_KEY}"
-        command: ["python", "-m", "llama_stack.core.server.server", "/etc/config/stack_run_config.yaml", "--port", "8321"]
+        command: ["llama", "stack", "run", "/etc/config/stack_run_config.yaml", "--port", "8321"]
         ports:
           - containerPort: 8321
         volumeMounts: