doc: add blurbs on zebra FPM interface and commands

Update documentation with some text on the zebra interface to the
optional Forwarding Path Manager component, and the related cli
commands.

Signed-off-by: Avneesh Sachdev <avneesh@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
1 file changed