[MLIR] Discourage people from copying the toy examples

[stephenneuendorffer]

Summary

The CMake structure of the toy example is non-standard. encourage people to
copy the standalone example instead.

Depends on D79841

[bondhugula]

mlir/examples/toy/Ch1/CMakeLists.txt

1

Nit: missing period at the end, and for the ones below.