Update landing page to include Llama (#6231)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6231

We should promote llama page more and demote the LLM Manual.

allow-large-files

bypass-github-export-checks
bypass-github-pytorch-ci-checks
bypass-github-executorch-ci-checks

Reviewed By: dvorjackz

Differential Revision: D64375318

fbshipit-source-id: a6b07de4735691b5230e51ec7bc1957ce9a4463f