Refine the LLM manual (focus on the debugging and profiling part) (#2952)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2952

* Some auto-formatting by my VSCode (remove extra spaces)
* Remove imports that have been imported in previous part of the doc
* Other minor changes to keep consistency across the doc
* Link a screenshot instead of using the raw table because the original table is illegible:
 {F1482781056}

Reviewed By: GregoryComer

Differential Revision: D55938344

fbshipit-source-id: 699abb9ebe1196ab73d90a3d08d60be7aa0d8688