Refactor out llama2 specific content out of Llama readme (#6359)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6359

Llama2 is "obselete", let's migrate to existing llama2 readme.md page

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Reviewed By: dvorjackz

Differential Revision: D64618486

fbshipit-source-id: 82b04aa93023dc021cb162986546d737d5e9f4dd