docs: Improve section about admin role binding
docs: Improve section about admin role binding

- Get GCP account name using a command
- Add link to GKE docs

Signed-off-by: Ilya Dmitrichenko <59b6733e47280dc0842a1e64fb2a9c742d686e46@gmail.com>

tklauser/cilium,tgraf/cilium,michi-covalent/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,tgraf/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium,tgraf/cilium,cilium-team/cilium,michi-covalent/cilium,michi-covalent/cilium,michi-covalent/cilium,cilium-team/cilium,cilium/cilium,tgraf/cilium,tgraf/cilium,cilium/cilium