Add CNX WAN Speed dashboard for the gateway speed tests

Per-gateway download/upload/ping from the hourly librespeed runs, with
each link's 7-day median overlaid so the WANSpeedDegraded alert
condition is visible at a glance. Label-driven — new sites appear
automatically once scraped.

docs/src/monitoring.md

@@ -52,6 +52,10 @@ there is picked up):
 - **CNX Uptime** (`uptime.json`) — per-host up/down status, current uptime,
   availability over the selected window, and up/down history. Label-driven, so
   every scraped host appears automatically.
+- **CNX WAN Speed** (`wan.json`) — per-gateway WAN speed from the hourly
+  librespeed runs: latest download/upload/ping, throughput history with each
+  link's 7-day median as reference, latency/jitter, and test status. See
+  [Site gateways](./gateways.md).
 - **parsedmarc** — DMARC aggregate/forensic report viewer. Auto-provisioned by
   the `parsedmarc` module (not from `dashboards/`); reads its own Elasticsearch
   datasource, not VictoriaMetrics. See [DMARC reports](#dmarc-reports) below.