[spp] Move SPP VF documentation

[Yasufumi Ogawa]

Hi all,

In SPP documentation, spp_vf and its siblings are described in chapter `SPP VF` dependently, but should be under each of other 
chapters as similar to spp_nfv. Spp_vf was almost completely separated from SPP and controlled from dedicated CLI, `spp_vf.py`. 
However, it is unified to SPP now. Documentation should be also unified to. I would like to start to move these documents after 
spp_pcap  Hideyuki has proposed is merged.

Thanks,
Yasufumi