doc: deduplicate extension descriptions into a new manpage

doc: deduplicate extension descriptions into a new manpage

iptables.8 and ip6tables.8 had pretty much the same content, with a few
protocol-specific deviations here and there. Not only did that bloat the
manpages, but it also made it harder to spot differences. Separate out
the extension descriptions into a new manpage, which conveniently
features differences next to one another (cf. REJECT).

Signed-off-by: Jan Engelhardt <jengelh@inai.de>