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>

docs/source/distributions/list_of_distributions.md

@@ -39,6 +39,13 @@ docker pull llama-stack/distribution-meta-reference-gpu
 
 **Guides:** [Meta Reference GPU Guide](self_hosted_distro/meta-reference-gpu)
 
+### 🖥️ Self-Hosted with NVIDA NeMo Microservices
+
+**Use `nvidia` if you:**
+- Want to use Llama Stack with NVIDIA NeMo Microservices
+
+**Guides:** [NVIDIA Distribution Guide](self_hosted_distro/nvidia)
+
 ### ☁️ Managed Hosting
 
 **Use remote-hosted endpoints if you:**