doc: add FRR/Linux configuration examples for EVPN

doc: add FRR/Linux configuration examples for EVPN

The existing EVPN documentation in bgp.rst does not provide a holistic
configuration, just examples of individual features, and doesn't give
an operator any idea of what a compatible Linux netdev configuration
might look like. This introduces evpn.rst which includes a sample
frr.conf and corresponding Linux interface config (via iproute2) that
an operator can use to setup a basic EVPN topology and model their
interface manager's config from.

This initial version of evpn.rst shows Linux netdev config for
traditional bridges (vlan_filtering=0) and traditional vxlan devices
(single VNI). Later changes to this file will cover the use of
VLAN-aware bridges (vlan_filtering=1), single VXLAN devices
(multi VNI), and eventually bonds (for EVPN-MH).

Eventually the plan is to move the existing EVPN content from bgp.rst
into evpn.rst, but for now let's get some user-facing documentation in
place for interface configs.

Signed-off-by: Trey Aspelund <taspelund@nvidia.com>