doc: fix mkdocs build warnings

- Add missing SVG For dhcpv6 example
 - Fix link to static routes in WireGuard doc
 - Add WireGuard to a new VPN sub-section in nav bar

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

doc/vpn-wireguard.md

@@ -141,7 +141,7 @@ for 192.168.2.0/24 through this peer.
 > [!NOTE]
 > When routing traffic to networks behind WireGuard peers, you also need
 > to configure static routes pointing to the WireGuard interface.  See
-> [Static Routes](networking.md#static-routes) for more information.
+> [Static Routes](networking.md#routing-support) for more information.
 
 ## Peer Configuration and Key Bags
 

mkdocs.yml

@@ -33,7 +33,9 @@ nav:
       - Quality of Service: qos.md
       - RMON Counters: eth-counters.md
       - Tunneling (L2/L3): tunnels.md
-      - VPN Tunnels: vpn.md
+      - VPN Tunnels:
+          - Overview: vpn.md
+          - WireGuard: vpn-wireguard.md
       - Wireless LAN (WiFi): wifi.md
   - Services:
       - Device Discovery: discovery.md