docs: add more instructions to Control Plane Auth using Custom Certs doc

[melsal13]

Description:

• link in before you begin section should point to cert-manager installation page instead of compatibility page
• First step of Configure Custom Certs for Control Plane section should be
  kubectl create namespace envoy-gateway-system
• should include values.yaml file for configuration set up in this page
• should include command on how to install eg with helm using values.yaml file
• missing verification to make sure everything is working correctly

Page: https://gateway.envoyproxy.io/docs/install/custom-cert/