Add Runframe integration to documentation

[niks918]

Adds Runframe to the Alertmanager Webhook Receiver integrations list. Runframe supports Prometheus Alertmanager through the standard webhook_configs receiver. Alertmanager can send alert webhook payloads to Runframe to create incidents, route to on-call responders, and publish status updates.

Example Alertmanager receiver configuration:

receivers:
  - name: runframe
    webhook_configs:
      - url: https://runframe.io/api/webhooks/<routing-key>
        send_resolved: true

Example Alertmanager webhook payload sent to Runframe:

{
  "receiver": "runframe",
  "status": "firing",
  "alerts": [
    {
      "status": "firing",
      "labels": {
        "alertname": "HighErrorRate",
        "severity": "critical",
        "service": "api"
      },
      "annotations": {
        "summary": "High error rate on API service",
        "description": "The API service is returning 5xx responses above the configured threshold."
      },
      "generatorURL": "https://prometheus.example.com/graph?g0.expr=...",
      "fingerprint": "abc123"
    }
  ],
  "externalURL": "https://alertmanager.example.com"
}

Docs-only change.