docs: mention xsrf issue in faq

docs/guide/faq.md

@@ -2,8 +2,13 @@
 outline: deep
 ---
 
-# FAQ
+# Frequently Asked Questions
 
-## Chapter 1
+[[toc]]
 
-## Chapter 2
+### _I have deployed Application Name alongside GeoNetwork, but somehow all the HTTP requests going to GeoNetwork end up failing with a 403 error, why?_
+
+There are several possible reasons for this:
+
+- The attempted requests necessitate authentication (e.g. creating a record) but the session of the current user has expired; in this case, the user should log in again.
+- The XSRF protection mechanism is not working correctly; this can be complicated to set up, please refer to [this part of the documentation](./deploy.md#authentication) to know more.