docs: add configuration troubleshooting reference docs (#2646)

Author: kayx23

docs/en/latest/config.json

@@ -39,6 +39,7 @@
             "items": [
                 "reference/api-reference",
                 "reference/example",
+                "reference/troubleshoot",
                 "reference/configuration-file",
                 "reference/ingress-annotation"
             ]

docs/en/latest/reference/troubleshoot.md

@@ -0,0 +1,56 @@
+---
+title: Configuration Troubleshooting
+slug: /reference/apisix-ingress-controller/configuration-troubleshoot
+description: Learn how to inspect and troubleshoot configuration translation and synchronization in APISIX Ingress Controller.
+---
+
+Troubleshooting is required if the applied behavior does not match expectations, such as routes not being created correctly, plugins not being applied, or services failing to route traffic.
+
+When you apply a Kubernetes resource—whether a Gateway API, Ingress, or APISIX CRD—the Ingress Controller translates it into ADC YAML, which is then applied to the gateway.
+
+This document explains how to inspect the translated ADC configurations in memory and check the configurations actually applied to the gateway.
+
+## Inspect Translated ADC Configurations
+
+APISIX Ingress Controller provides a browser-accessible debug API that displays the translated ADC configurations, derived from the last applied Gateway API, Ingress, and APISIX CRD resources, in JSON format. It helps inspect the __in-memory state before the configurations are synchronized with the gateway__.
+
+To use the debug API, configure these values in the ingress controller's [configuration file](./configuration-file.md):
+
+```yaml title="config.yaml"
+enable_server: true             # Enable the debug API server
+server_addr: "127.0.0.1:9092"   # Server address
+```
+
+These values are not yet available in the Helm chart. To apply the changes, modify the ConfigMap and restart the controller Deployment.
+
+Once the debug API is enabled, you can access it by forwarding the controller pod’s port to your local machine:
+
+```shell
+kubectl port-forward pod/<your-apisix-ingress-controller-pod-name> 9092:9092 &
+```
+
+You can now access the debug API in browser at `127.0.0.1:9092/debug` and inspect the translated resources by resource type, such as routes and services.
+
+## Inspect Synchronized Gateway Configurations
+
+To inspect the configurations synchronized to the gateway, you can use the Admin API.
+
+First, forward the Admin API's service port to your local machine:
+
+```shell
+kubectl port-forward service/apisix-admin 9180:9180 &
+```
+
+If you are using APISIX in standalone mode, you can send a request to `/apisix/admin/configs` to view all configurations synchronized to the gateway:
+
+```shell
+curl "http://127.0.0.1:9180/apisix/admin/configs" -H "X-API-KEY: ${ADMIN_API_KEY}"
+```
+
+If you are using APISIX with etcd, you can send a request to `/apisix/admin/<resource>` to view the synchronized configurations of specific resources. For instance, to view the route configuration:
+
+```shell
+curl "http://127.0.0.1:9180/apisix/admin/routes" -H "X-API-KEY: ${ADMIN_API_KEY}"
+```
+
+For reference, see [Admin API](https://apisix.apache.org/docs/apisix/admin-api/).