Add docs for allowlist_external_urls (#13805)

Co-authored-by: Franck Nijhof <git@frenck.dev>

source/_docs/configuration/basic.markdown

@@ -21,6 +21,8 @@ homeassistant:
   whitelist_external_dirs:
     - /usr/var/dumping-ground
     - /tmp
+  allowlist_external_urls:
+    - "http://images.com/image1.png"
 ```
 
 NOTE: You will not be able to edit anything in Configuration -> General in the UI if you are using YAML configuration for any of the following: name, latitude, longitute, elevation, unit_system, temperature_unit, time_zone, external_url, internal_url.
@@ -78,6 +80,10 @@ whitelist_external_dirs:
   description: List of folders that can be used as sources for sending files.
   required: false
   type: list
+allowlist_external_urls:
+  description: List of external URLs that can be fetched. URLs can match specific resources (e.g., `http://10.10.10.12/images/image1.jpg`) or a relative path that allows access to resources within it (e.g., `http://10.10.10.12/images` would allow access to anything under that path)
+  required: false
+  type: list
 {% endconfiguration %}
 
 ## Reload Core Service