docs: Add summary of bird integration
docs: Add summary of bird integration

This page previously lacked the core explanation of the roles that
either Cilium or bird perform in this guide: That Cilium manages
in-cluster network connectivity while bird advertises the pod ranges
outside the cluster.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

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