Add note within ux documentation that further changes should be made within...

Add note within ux documentation that further changes should be made within the design.gitlab project

changelogs/unreleased/ux-guide-deprecation.yml

+---
+title: Add note within ux documentation that further changes should be made within
+  the design.gitlab project
+merge_request:
+author:
+type: deprecated

doc/development/ux_guide/index.md

+> We are in the process of transferring UX documentation to the [design.gitlab.com](https://gitlab.com/gitlab-org/design.gitlab.com) project. Any updates to these docs should be made in that project. If documentation does not yet exist within [design.gitlab.com](https://gitlab.com/gitlab-org/design.gitlab.com), [create an issue](https://gitlab.com/gitlab-org/design.gitlab.com/issues) and merge request to add your new changes.
+
 # GitLab UX Guide
 
 The goal of this guide is to provide standards, principles and in-depth information to design beautiful and effective GitLab features. This will be a living document, and we welcome contributions, feedback and suggestions.