fix(ai-gateway): Add faq for using custom PII service (#3914)

Author: tomek-labuk

app/_kong_plugins/ai-sanitizer/index.md

@@ -39,6 +39,16 @@ categories:
 tags:
   - ai
 
+faqs:
+  - q: Can I use a custom PII anonymization service instead of Kong's AI PII Anonymizer?
+    a: |
+      To use a custom PII service, configure the [Request Callout](/plugins/request-callout/) or [Datakit](/plugins/datakit/) plugin to:
+      1. Send the request payload to your PII service.
+      2. Receive the sanitized response.
+      3. Forward the transformed payload to the upstream service.
+
+      Your custom service must implement Kong's PII service interface if you want to use the AI PII Sanitizer plugin with it.
+
 related_resources:
   - text: Use AI PII Sanitizer plugin to protect sensitive information in requests
     url: /how-to/protect-sensitive-information-with-ai/