fix: Restore the nvidia distro (#2639)

# What does this PR do?

The `nvidia` distro was previously collapsed into the `starter` distro.
However, the `nvidia` distro was setup specifically to use NVIDIA NeMo
microservices as providers for all APIs and not just inference, which
means it was doing quite a bit more than what the `starter` distro
covers today.

We should work with our friends at NVIDIA to determine the best place to
maintain this distro long-term, but for now this restores the `nvidia`
distro and its docs back to where they were so that things continue to
work for their users.

## Test Plan

I ensure the `nvidia` distro could build, and run at least to the point
of complaining that I didn't provide the necessary API keys.

```
uv run llama stack build --template nvidia --image-type venv
uv run llama stack run llama_stack/templates/nvidia/run.yaml
```

I also made sure the docs website built and looks reasonable, with the
`nvidia` distro docs at the same URL it was previously (because it has
incoming links from official NVIDIA NeMo docs, among other places).

```
uv run --group docs sphinx-autobuild docs/source docs/build/html --write-all
```

Signed-off-by: Ben Browning <bbrownin@redhat.com>

llama_stack/templates/nvidia/build.yaml

@@ -0,0 +1,29 @@
+version: 2
+distribution_spec:
+  description: Use NVIDIA NIM for running LLM inference, evaluation and safety
+  providers:
+    inference:
+    - remote::nvidia
+    vector_io:
+    - inline::faiss
+    safety:
+    - remote::nvidia
+    agents:
+    - inline::meta-reference
+    telemetry:
+    - inline::meta-reference
+    eval:
+    - remote::nvidia
+    post_training:
+    - remote::nvidia
+    datasetio:
+    - inline::localfs
+    - remote::nvidia
+    scoring:
+    - inline::basic
+    tool_runtime:
+    - inline::rag-runtime
+image_type: conda
+additional_pip_packages:
+- aiosqlite
+- sqlalchemy[asyncio]