Documentation for user impersonation via kubeconfig

**This is a Feature Request**

**What would you like to be added**
Currently, only the impersonation of users via headers and kubectl flags is documented. It would be helpful to also explain and describe the possibility to configure user impersonation via kubeconfig.

**Why is this needed**
Its sometimes helpful to configure the user impersonation via kubeconfig, e.g. for external tooling (Example: https://github.com/istio/istio/issues/22365#issuecomment-610419899) but currently you need to know that it is possible and check the API spec for further information.

**Comments**
- https://github.com/istio/istio/issues/22365#issuecomment-610419899)
- https://github.com/kubernetes/client-go/blob/b69cda3e4e09e1b956d42709fbadf12e6c0f24b7/tools/clientcmd/api/types.go#L131-L140

Start with the existing Kubernetes documentation for user impersonation and compare it with the kubeconfig API fields in client-go/tools/clientcmd/api/types.go, lines 131-140. Document how user impersonation can be configured in kubeconfig, including the relevant external-tool use case, and make the new guidance discoverable alongside the existing impersonation instructions.

Written by the indexing model from the issue text.