Add the Tech Writers table in the MR Documentation template

Mention where one can find the designated tech writer for
a group/stage.

.gitlab/merge_request_templates/Documentation.md

 <!-- Follow the documentation workflow https://docs.gitlab.com/ee/development/documentation/workflow.html -->
 <!-- Additional information is located at https://docs.gitlab.com/ee/development/documentation/ -->
+<!-- To find the designated Tech Writer for the stage/group, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers -->
 
 <!-- Mention "documentation" or "docs" in the MR title -->
 <!-- For changing documentation location use the "Change documentation location" template -->