Update examples/README.md with Llama 3 and names (#3275)

Summary:
- Added Llama 3 8B
- Added llm_manual in the list
- changed name from Extensa to Cadence

Pull Request resolved: https://github.com/pytorch/executorch/pull/3275

Reviewed By: Gasoonjia

Differential Revision: D56524960

Pulled By: iseeyuan

fbshipit-source-id: 2b4464028fe3cdf3c2b524d233fa3e87b2561dda