docs(msgraph): webhook listener setup page + env var reference

Second docs slice shipped alongside the webhook listener code so users
can actually wire up the endpoint the moment this PR lands.

- website/docs/user-guide/messaging/msgraph-webhook.md: new page
  covering what the listener is (change-notification ingress, distinct
  from the teams chat adapter), quick-start YAML + env-var config,
  full config table, security hardening (clientState + timing-safe
  compare, source-IP allowlisting against Microsoft's published egress
  ranges, TLS termination at the reverse proxy, response hygiene),
  status-code table, troubleshooting, and cross-links to the Azure
  app registration guide.

- website/docs/reference/environment-variables.md: new Microsoft
  Graph Webhook Listener subsection with MSGRAPH_WEBHOOK_ENABLED,
  _PORT, _CLIENT_STATE, _ACCEPTED_RESOURCES, _ALLOWED_SOURCE_CIDRS.

- website/sidebars.ts: wire the new page into Messaging Platforms,
  right after the teams chat adapter so the two related pages are
  adjacent in the sidebar.

The pipeline runtime / operator CLI / outbound delivery pages still
land with their matching PRs. With this PR merged, an operator can get
the listener running end-to-end, register a Graph subscription
manually, and receive validation handshake plus notification POSTs
against the configured client_state.

Verified via npm run build: new page routes at
/docs/user-guide/messaging/msgraph-webhook, sidebar wires correctly,
no new warnings or errors.

website/sidebars.ts

@@ -136,6 +136,7 @@ const sidebars: SidebarsConfig = {
         'user-guide/messaging/qqbot',
         'user-guide/messaging/yuanbao',
         'user-guide/messaging/teams',
+        'user-guide/messaging/msgraph-webhook',
         'user-guide/messaging/open-webui',
         'user-guide/messaging/webhooks',
       ],