Update iOS Llama demo app readme docs (#5359)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5359

- Revamp and standardize the readme docs structure for better clarity.
- Added different delegate instructions for ios.
- Added some screenshots to improve readme clarity.

Reviewed By: larryliu0820

Differential Revision: D62660106

fbshipit-source-id: b84e94b85b5cd295846a567dcf7c4c271619904d