Document redir iptables setup

[TurboTheTurtle]

Summary

• document how Linux iptables transparent redirection should target sslocal --protocol redir
• add a minimal TCP-only local redirect example with outbound_fwmark loop prevention
• point users to the existing TCP/UDP scripts and clarify how outbound_proxy relates to HTTP/SOCKS upstreams

Validation

• git diff --check

Closes #2134