Update Cerebras docs to include header (#704)

# What does this PR do?

I noticed that the documentation for other providers have this header,
so I have added it to the Cerebras docs too.
```
---
orphan: true
---

# TGI Distribution

```{toctree}
:maxdepth: 2
:hidden:

self
    ```
```

This also fixes a typo in README.md where the link to the Cerebras docs included an extra `getting_started` section.

I did notice however that https://hub.docker.com/r/llamastack/distribution-cerebras still does not exist. How do I get the Cerebras Docker image uploaded?

cc: @ashwinb @raghotham 

## Before submitting

- [X] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] Ran pre-commit to handle lint / formatting issues.
- [ ] Read the [contributor guideline](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

docs/source/distributions/self_hosted_distro/cerebras.md

@@ -1,5 +1,15 @@
+---
+orphan: true
+---
 # Cerebras Distribution
 
+```{toctree}
+:maxdepth: 2
+:hidden:
+
+self
+```
+
 The `llamastack/distribution-cerebras` distribution consists of the following provider configurations.
 
 | API | Provider(s) |