Add network policy document

docs/user-guides/network-policy.md

@@ -0,0 +1,55 @@
+# Network Policy
+
+[Network Policy](https://kubernetes.io/docs/concepts/services-networking/network-policies/) is a Kubernetes feature that controls ingress and egress network traffic for pods. In Vald, you can set network policies as follows.
+
+> Please note that [prerequisites](https://kubernetes.io/docs/concepts/services-networking/network-policies/#prerequisites) are required for using network policies. Even if you configure the following settings in a cluster that does not meet the prerequisites, network policies will not be effective.
+
+# Network Policy in Vald
+
+To enable network policies in a Vald cluster, set `defaults.networkPolicy.enabled` to `true` as follows:
+
+```yaml
+defaults:
+  networkPolicy:
+    enabled: true
+```
+
+This sets the following ingress/egress rules between Vald components (these are the minimum required rules for a Vald cluster to work).
+
+| from / to      | agent | discoverer | filter gateway | lb gateway | index manager | kube-system |
+| -------------- | ----- | ---------- | -------------- | ---------- | ------------- | ----------- |
+| agent          | N/A   | ⛔         | ⛔             | ⛔         | ⛔            | ✅          |
+| discoverer     | ⛔    | N/A        | ⛔             | ⛔         | ⛔            | ✅          |
+| filter gateway | ⛔    | ⛔         | N/A            | ✅         | ⛔            | ✅          |
+| lb gateway     | ✅    | ✅         | ⛔             | N/A        | ⛔            | ✅          |
+| index manager  | ✅    | ✅         | ⛔             | ⛔         | N/A           | ✅          |
+
+# Add a user custom Network Policy
+
+There may be cases where you want to connect a Vald cluster to external components. Specifically, for the following cases:
+
+- Enable egress to `OpenTelemetryCollector` to use [observability features](https://vald.vdaas.org/docs/user-guides/observability-configuration/)
+- Enable egress to an external filter component to use [filtering features](https://vald.vdaas.org/docs/user-guides/filtering-configuration/).
+
+To handle such cases, Vald allows you to set user custom network policies using the `defaults.networkPolicy.custom` field as follows:
+
+```yaml
+defaults:
+  networkPolicy:
+    enabled: true
+    custom:
+      ingress:
+        - from:
+            - podSelector:
+                matchLabels:
+                  app.kubernetes.io/name: pyroscope
+      egress:
+        - to:
+            - podSelector:
+                matchLabels:
+                  app.kubernetes.io/name: opentelemetry-collector-collector
+```
+
+Please write down the same notation as the `ingress/egress` field of [NetworkPolicy resource](https://kubernetes.io/docs/concepts/services-networking/network-policies/#networkpolicy-resource) in our `custom` field.
+
+> Currently, these custom network policies are applied to all Vald components.