docs: add istio docs (#15733)

closes https://github.com/coder/coder/issues/11821

docs/admin/integrations/istio.md

@@ -0,0 +1,35 @@
+# Integrate Coder with Istio
+
+Use Istio service mesh for your Coder workspace traffic to implement access
+controls, encrypt service-to-service communication, and gain visibility into
+your workspace network patterns. This guide walks through the required steps to
+configure the Istio service mesh for use with Coder.
+
+While Istio is platform-independent, this guide assumes you are leveraging
+Kubernetes. Ensure you have a running Kubernetes cluster with both Coder and
+Istio installed, and that you have administrative access to configure both
+systems. Once you have access to your Coder cluster, apply the following
+manifest:
+
+```yaml
+apiVersion: networking.istio.io/v1alpha3
+kind: EnvoyFilter
+metadata:
+  name: tailscale-behind-istio-ingress
+  namespace: istio-system
+spec:
+  configPatches:
+    - applyTo: NETWORK_FILTER
+      match:
+        listener:
+          filterChain:
+            filter:
+              name: envoy.filters.network.http_connection_manager
+      patch:
+        operation: MERGE
+        value:
+          typed_config:
+            "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
+            upgrade_configs:
+              - upgrade_type: derp
+```