GKE cniBinDir documentation removed

[howardjohn]

istio/istio#53849 (comment) -- refactoring removed any documentation of cniBinDir setting for GKE. https://istio.io/latest/docs/ambient/install/platform-prerequisites/#google-kubernetes-engine-gke is missing it and https://istio.io/latest/docs/setup/additional-setup/cni/ links only there

cc @bleggett

[bleggett]

This is already fixed (the right way) in #16035 but wasn't backported.

I'll do a 1.24 only tweak docs backport that will be superceded in the next release with the platform profile anyway - that will automatically also cover this case (which only happens today if you aren't using either of the two recommended install methods of istioctl or helm)

[bleggett]

There are probably other template-install-only gotchas we have, but we do not and have not ever really consistently documented them.