doc (components/AP Gateway): document new http_sign_get configuration option.

doc/components.rst

@@ -244,6 +244,13 @@ All options are to be set in ``[component ap-gateway]`` section.
 
   **default**: ``true``
 
+``http_sign_get``
+  A boolean value indicating if HTTP ``GET`` request made to AP implementations must be
+  signed. Some AP implementations require those requests to be signed, notably Mastodon
+  when "secure mode" is activated.
+
+  **default**: ``true``
+
 ``html_redirect_dict``
   A dictionary used to redirect HTTP requests when it's not an ActivityPub request (i.e.
   when the ``Accept`` header is not set to ``application/json``). Several ActivityPub