Update and fix docs (namespaces, consistency) (#6084)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/6084

Audit all instances of `\bexec_aten::` and `\btorch::` under `docs/`, updating where appropriate.

The only remaining `torch::` instances are for kernels, which I didn't get a chance to migrate before v0.4.0.

Also:
- Update the LLM Manual code to be consistent between the doc and main.cpp
- Fix some LLM Manual issues: point to the latest release, and "main.cpp" instead of "main.h"

Reviewed By: mergennachin, Gasoonjia, Olivia-liu

Differential Revision: D64152344

fbshipit-source-id: 2f6582429d5e3ef285b728350f937247996bb454