doc: add new website at netfilter.org to doxygen
doc: add new website at netfilter.org to doxygen

Signed-off-by: Pablo Neira Ayuso <707d14912bb250caf67dfe0ea4035681fbfc4f56@netfilter.org>

remfalc/vyt-libmnl,vyatta/libmnl,vyos/libmnl,vyatta/libmnl,fqrouter/libmnl,greearb/libmnl-ct,vyos/libmnl,chamaken/libmnl,cyclops8456/libmnl,vyos/libmnl,cyclops8456/libmnl,cyclops8456/libmnl,chamaken/libmnl,greearb/libmnl-ct,remfalc/vyt-libmnl,Distrotech/libmnl,threatstack/libmnl,threatstack/libmnl,fqrouter/libmnl,Distrotech/libmnl