Add more instructions to Xcode setup (#5757)

Summary:
I ran into the following error during `Build Cmake Dependencies` phase:

```
cmake not found, please install cmake.
```

It was not clear how to install cmake. This PR adds the following instructions:

```
cmake not found, please install cmake.
1. Download Cmake.app from https://cmake.org/download with version > 3.19.
2. Install it to Applications/ folder and run sudo /Applications/CMake.app/Contents/bin/cmake-gui --install to install CMake commandline tools.
```

<img width="1130" alt="Screenshot 2024-09-30 at 9 46 04 AM" src="https://github.com/user-attachments/assets/3aa220c6-27da-4b8d-a7b0-3bd2f8e6b5a3">

Pull Request resolved: https://github.com/pytorch/executorch/pull/5757

Reviewed By: shoumikhin

Differential Revision: D63643170

Pulled By: larryliu0820

fbshipit-source-id: c565a0f1d9e0001e9b43cac4e68e64d270f48018