Clean up annotations.md; extract default backend from miscellaneous

docs/user-guide/default-backend.md

@@ -0,0 +1,17 @@
+# Default backend
+
+The default backend is a service which handles all URL paths and hosts the nginx controller doesn't understand
+(i.e., all the requests that are not mapped with an Ingress).
+
+Basically a default backend exposes two URLs:
+
+- `/healthz` that returns 200
+- `/` that returns 404
+
+!!! example
+    The sub-directory [`/images/404-server`](https://github.com/kubernetes/ingress-nginx/tree/master/images/404-server)
+    provides a service which satisfies the requirements for a default backend.
+    
+!!! example
+    The sub-directory [`/images/custom-error-pages`](https://github.com/kubernetes/ingress-nginx/tree/master/images/custom-error-pages)
+    provides an additional service for the purpose of customizing the error pages served via the default backend.