Refine LLM getting started guide for uniformity, fix critical errors (#2911)

Summary:
Update the LLM getting started guide for uniform tone and tense. Informally following the Google developer documentation style guide: https://developers.google.com/style. Also, resolve a number of outstanding issues with incorrect or misleading documentation and steps.

For reference, here are links to the current and proposed LLM guide:
https://docs-preview.pytorch.org/pytorch/executorch/2911/llm/getting-started.html (proposed)
https://pytorch.org/executorch/main/llm/getting-started.html (live)

Pull Request resolved: https://github.com/pytorch/executorch/pull/2911

Reviewed By: Gasoonjia, byjlw

Differential Revision: D55867181

Pulled By: GregoryComer

fbshipit-source-id: 5e865eaa4a0ae52845963b15c221a3d272431448