Add some basic rpc api docs

Add some basic rpc api docs

The devref docs had a placeholder file for rpc API docs.  Now that
both a client side and server side interface have been converted from
the rpc compat layer to oslo.messaging APIs, add some docs that give
an overview of what the client and server sides look like.

Also include a section that describes the code layout of where you can
find client and server api implementations in the neutron code base.
It starts by discussing the DHCP agent related APIs.

Part of blueprint drop-rpc-compat.

Change-Id: Ib391958252077365a81bcb881ab27a078f71fdec