Update LLaMA docs with arxiv link
# What does this PR do?
Fixes #23186 

Adds arxiv link for "LLaMA: Open and Efficient Foundation Language Models" (https://arxiv.org/abs/2302.13971) to LLaMA model docs.

## Who can review?
@sgugger, @stevhliu and @MKhalusova
_The documentation is not available anymore as the PR was closed or merged._