OpenCL set-up documentation points to files that do not exist

[bgall]

Section 14.3.1 of the cmdstan guide explains how to check for OpenCL runtimes etc. and points to pre-built binary for Windows that can be downloaded from this GitHub repo. Unfortunately, it looks like these files are often unavailable (links to files are dead) and this has been an issue since 2000: https://github.com/Oblomov/clinfo/issues/45

Not sure if these links should be removed from the documentation or an alternative explanation/tool might be provided. For example: https://opencl.gpuinfo.org/