Readme docs update (#5695)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5695

- rename XNNPACK
- remove duplicate executorch setup instructions
- remove tokenizer conversion step (since iOS supports both .bin and .model)
- move model copying section to after xcode setup

Reviewed By: cmodi-meta, kirklandsign

Differential Revision: D63479985

fbshipit-source-id: bd1030588fa997f26c0c8da59f5850b06141aa43