Update MPS documentation; add helper script to build mps_executor_runner (#3324)

Summary:
**Summary of changes**:
- Update MPS documentation to reflect all changes since previous release
- Add helper script to build `mps_executor_runner`

**Testing**:
- Verified that mps_executor_runner builds correctly:
```
./examples/apple/mps/scripts/build_mps_executor_runner.sh
/examples/apple/mps/scripts/build_mps_executor_runner.sh --Debug
```
Verified that the docs are building correctly:
```
cd docs
make html
```

cc shoumikhin, cccclai

Pull Request resolved: https://github.com/pytorch/executorch/pull/3324

Reviewed By: shoumikhin

Differential Revision: D56535774

Pulled By: cccclai

fbshipit-source-id: 5974795732dbe1089e3d63cd1b618cadf7a2573e