Update documentation for cmake selective build features

[lucylq]

Now that @BujSet has added functionality for dtype selective build, we can update the documentation with usage instructions/examples.

1. Model API
2. Dtype selective build API

https://docs.pytorch.org/executorch/stable/kernel-library-selective-build.html
https://github.com/pytorch/executorch/tree/main/examples/selective_build

cc @larryliu0820 @jathu