[doc] Add IPv6 overlay addressing documentation

[lixmal]

• Add new IPv6 overlay addressing page covering enabling, address assignment, DNS, firewall rules, exit nodes, and CLI usage
• Update exit nodes page with IPv6 support section, replace WireGuard references with generic "tunnel interface"
• Add --disable-ipv6 and --ipv6 flags to CLI reference

Note: The version requirement (v0.68.0) is a placeholder and may need adjustment before merge.

Screenshots

IPv6 Overlay Addressing page

Exit Nodes (IPv6 section)

CLI reference (new flags)