Add proxy ARP external connectivity documentation

[Copilot]

Adds documentation for using proxy ARP on a Linux gateway to provide external connectivity to lab devices without NAT or routing protocols, as discussed in #2861.

Changes

• New section: "Using Proxy ARP for External Connectivity" in docs/example/external.md
  • Solution overview with 192.18.42.0/24 example subnet
  • Sample topology with Linux gateway node and external uplink
  • User defaults configuration via netlab defaults command
  • Linux server proxy ARP setup commands

Example Topology

defaults.device: eos

nodes:
  gw:
    device: linux
    role: gateway
  r1:
  r2:

links:
- gw:
    ipv4: 192.18.42.1/24
  r1:
    ipv4: 192.18.42.10/24
  r2:
    ipv4: 192.18.42.11/24
  clab:
    uplink: eth1

Configuration Commands

# Configure addressing pool
netlab defaults --user addressing.external.ipv4=192.18.42.0/24
netlab defaults --user addressing.external.start=10

# Enable proxy ARP on gateway
sudo sysctl -w net.ipv4.conf.eth1.proxy_arp=1
sudo ip route add 192.18.42.0/24 dev eth2
sudo sysctl -w net.ipv4.ip_forward=1

Original prompt

Based on discussion in #2861, add a new section to docs/example/external.md to describe this approach. The section should include a brief overview of the solution (assuming 192.18.42.0/24 is the external subnet), a sample lab topology, the recommended changes to user defaults (use "netlab defaults" command to make them) and the proxy arp configuration on the Linux server

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