Fix#1506: restore the VLAN interface documentation that was lost when
networking.md was split into per-topic pages. Content adapted from the
v25.11.0 networking.md VLAN section, plus a new "Stacked (Q-in-Q)"
section showing `eth0.10.20`-style nesting and the layered `show
interface` rendering of the parent chain.
Update the interface-type table in networking.md to point at the new
vlan.md instead of the dangling ethernet.md#vlan-interfaces anchor.
ChangeLog: document the user-facing changes on this branch (YANG model
upgrade, layered show interface output, auto-negotation typo fix) and
the new VLAN documentation.
Signed-off-by: Joachim Wiberg <troglobit@gmail.com>
