netfilter: doc: Add nf_tables part in tproxy.txt

Recently, transparent proxy support has been added to nf_tables so that
this document should be updated with the new information.

- Nft commands are added as alternatives to iptables ones.
- The link for a patched iptables is removed as it is already part of
  the mainline iptables implementation (and the link is dead).
- tcprdr is added as an example implementation of a transparent proxy

Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Florian Westphal <fw@strlen.de>
Cc: KOVACS Krisztian <hidden@sch.bme.hu>
Cc: Pablo Neira Ayuso <pablo@netfilter.org>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Máté Eckl <ecklm94@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>