Remove or merge llm/llama-3/hello-llama-3.ipynb

[ccozad]

llm/llama-3/hello-llama-3.ipynb covers the same ground as llm/llama-3/hello-world/ (which has a detailed README + hello.py). The notebook duplicates and adds no new content.

Work

Pick one:

1. Delete the notebook — simplest. The hello-world directory is a strict superset.
2. Merge any unique content into the hello-world README, then delete.

Either way, no top-level README link change is needed because the current top-level README does not link to the notebook directly (it points at hello-llama-3.ipynb once via Local Llama 3 chat demo — that link should be updated to point at hello-world/ after this issue lands).

See docs/analysis/02-notebook-conversion.md.