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/__init__.py

@@ -0,0 +1,7 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+from .nvidia import get_distribution_template  # noqa: F401