cgmanifest.json: Move to a subdirectory

[StephanTLavavej]

This is a machine-readable file for Microsoft-internal purposes. It must remain in our tree, but there's no reason for it to be at the top level, where it pushes the readme down. Moving it to the stl subdirectory would be an improvement (as it covers the code within). The tools that read this don't care about its location at all.

[BillyONeal]

Can it go into docs instead?

I didn't want to put it in the stl directory in case we add other parts like vcruntime to this repo in the future.

[StephanTLavavej]

That seems like a good place, yes.