update plugins doc (#1305)

plugins/wasm-go/extensions/opa/README_EN.md

@@ -0,0 +1,58 @@
+---
+title: OPA
+keywords: [higress,opa]
+description: OPA policy control plugin configuration reference
+---
+## Function Description
+This plugin implements `OPA` policy control.
+
+## Running Attributes
+Plugin Execution Phase: `Authentication Phase`  
+Plugin Execution Priority: `225`
+
+## Configuration Fields
+| Field            | Data Type   | Required | Default Value | Description                                   |
+|------------------|-------------|----------|---------------|-----------------------------------------------|
+| policy           | string      | Required | -             | OPA Policy                                    |
+| timeout          | string      | Required | -             | Timeout setting for access                    |
+| serviceSource    | string      | Required | -             | k8s, nacos, ip, route                         |
+| host             | string      | Optional | -             | Service host (required if serviceSource is `ip`) |
+| serviceName      | string      | Optional | -             | Service name (required if serviceSource is `k8s,nacos,ip`) |
+| servicePort      | string      | Optional | -             | Service port (required if serviceSource is `k8s,nacos,ip`) |
+| namespace        | string      | Optional | -             | Namespace (required if serviceSource is `k8s,nacos`) |
+
+## Configuration Example
+```yaml
+serviceSource: k8s
+serviceName: opa
+servicePort: 8181
+namespace: higress-backend
+policy: example1
+timeout: 5s
+```
+
+## OPA Service Installation Reference
+### Start OPA Service
+```shell
+docker run -d --name opa -p 8181:8181 openpolicyagent/opa:0.35.0 run -s
+```
+
+### Create OPA Policy
+```shell
+curl -X PUT '127.0.0.1:8181/v1/policies/example1' \
+  -H 'Content-Type: text/plain' \
+  -d 'package example1
+import input.request
+default allow = false
+allow {
+    # HTTP method must GET
+    request.method == "GET"
+}'
+```
+
+### Query Policy
+```shell
+curl -X POST '127.0.0.1:8181/v1/data/example1/allow' \
+  -H 'Content-Type: application/json' \
+  -d '{"input":{"request":{"method":"GET"}}}'
+```