Minor documentation improvements in the CodeGen README

[dmsuehir]

Description

This PR makes minor documentation updates for things that I noticed when running this example:

• There is a "Guide" link that took me to a directory without a README.md, so I changed this to link to the Gaudi and Xeon READMEs that have instructions for building the docker images from source
• The instructions have me set my HF token env var, but there is no mention of what model is being used and I didn't previously have access CodeLlama.
• Fixed a typo in the Gaudi driver version

Type of change

List the type of change like below. Please delete options that are not relevant.

• Others (enhancement, documentation, validation, etc.)

Dependencies

N/A

Tests

N/A

[ashahba]

LGTM!