chore: rename templates to distributions (#3035)

As the title says. Distributions is in, Templates is out.

`llama stack build --template` --> `llama stack build --distro`. For
backward compatibility, the previous option is kept but results in a
warning.

Updated `server.py` to remove the "config_or_template" backward
compatibility since it has been a couple releases since that change.

llama_stack/distributions/postgres-demo/build.yaml

@@ -0,0 +1,25 @@
+version: 2
+distribution_spec:
+  description: Quick start template for running Llama Stack with several popular providers
+  providers:
+    inference:
+    - provider_type: remote::vllm
+    - provider_type: inline::sentence-transformers
+    vector_io:
+    - provider_type: remote::chromadb
+    safety:
+    - provider_type: inline::llama-guard
+    agents:
+    - provider_type: inline::meta-reference
+    telemetry:
+    - provider_type: inline::meta-reference
+    tool_runtime:
+    - provider_type: remote::brave-search
+    - provider_type: remote::tavily-search
+    - provider_type: inline::rag-runtime
+    - provider_type: remote::model-context-protocol
+image_type: venv
+additional_pip_packages:
+- asyncpg
+- psycopg2-binary
+- sqlalchemy[asyncio]