Use relative links in llm/getting-started.md (#3244)

Summary:
Use relative markdown links instead of full URLs. This way, the docs will always point to a consistent branch.

Pull Request resolved: https://github.com/pytorch/executorch/pull/3244

Test Plan: Clicked on all modified links in the rendered docs preview: https://docs-preview.pytorch.org/pytorch/executorch/3244/llm/getting-started.html

Reviewed By: Gasoonjia

Differential Revision: D56479234

Pulled By: dbort

fbshipit-source-id: 45fb25f017c73df8606c3fb861acafbdd82fec8c