Highlight dependency on shader files in examples

[kornelski]

The examples won't work when copy-pasted to another project, without also copying their shader files. This change adds constants at the top of the files to bring attention to the dependencies.

Follow up to #13624

[killercup]

If this is what we want, we might also want to add a small CI check to ensure all shader/asset paths are defined in constants (in a follow-up).

[alice-i-cecile]

Thanks for tackling this: I know these tasks can be really tedious.