Update Android ExecuTorch Llama demo app readme docs (#5364)

Summary:
* Revamp and standardize the readme docs structure for better clarity.
* Added XNNPACK and MTK delegate instructions for Android.
* Added app screenshots for user instruction.
* Added a `docs` directory to host README materials

Pull Request resolved: https://github.com/pytorch/executorch/pull/5364

Reviewed By: larryliu0820

Differential Revision: D62667589

Pulled By: cmodi-meta

fbshipit-source-id: 024d989e802886f21e0df2d561bd8aa49b1ab90e