[doc][rfe] Improve DPDK docs

[bouabid amine]

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It is not clear in the doc how to create the dpdk_bridge interface/tunnel and how to configure neutron_external_interface?!

I tried the following:
1- I installed the OVS and OVS-DPDK package directly on the host system
2- I created the dpdk_bridge using the ovs-vsctl command
3- I attached the bridge to the physical interface (i prepared a bond interface composed of two physical nics)
4- I affected fake ip addresses to dpdk_bridge and bond0 interface, unless kolla-ansible returns an error!

With this, the deployment is achieved, but when creating instances, i am experiencing some troobles:
- i can't login the instance (i gave the cloud-init script)
- i can't access the instance via ssh (i created an ssh pair)
- when i create an instance with page_mem, the process is aborted because it oes not fit the numa architecture!

-----------------------------------
Release: 7.2.2.dev14 on 2020-09-14 11:39
SHA: 1afbbeb874a6b4529c3df8785f2d822a96085c84
Source: https://git.openstack.org/cgit/openstack/kolla-ansible/tree/doc/source/reference/networking-guide.rst
URL: https://docs.openstack.org/kolla-ansible/rocky/reference/networking-guide.html