Simplify the 'Implement design & UI elements' section in CONTRIBUTING

Signed-off-by: Rémy Coutable <remy@rymai.me>

CONTRIBUTING.md

@@ -91,19 +91,7 @@ This was inspired by [an article by Kent C. Dodds][medium-up-for-grabs].
 ## Implement design & UI elements
-### Design reference
-
-The GitLab design reference can be found in the [gitlab-design] project.
-The designs are made using Antetype (`.atype` files). You can use the
-[free Antetype viewer (Mac OSX only)] or grab an exported PNG from the design
-(the PNG is 1:1).
-
-The current designs can be found in the [`gitlab8.atype` file].
-
-### UI development kit
-
-Implemented UI elements can also be found at https://gitlab.com/help/ui. Please
-note that this page isn't comprehensive at this time.
+Please see the [UI Guide for building GitLab].
 ## Issue tracker
@@ -489,7 +477,5 @@ available at [http://contributor-covenant.org/version/1/1/0/](http://contributor
 [doc-styleguide]: doc/development/doc_styleguide.md "Documentation styleguide"
 [scss-styleguide]: doc/development/scss_styleguide.md "SCSS styleguide"
 [newlines-styleguide]: doc/development/newlines_styleguide.md "Newlines styleguide"
-[gitlab-design]: https://gitlab.com/gitlab-org/gitlab-design
-[free Antetype viewer (Mac OSX only)]: https://itunes.apple.com/us/app/antetype-viewer/id824152298?mt=12
-[`gitlab8.atype` file]: https://gitlab.com/gitlab-org/gitlab-design/tree/master/current/
+[UI Guide for building GitLab]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/ui_guide.md
 [license-finder-doc]: doc/development/licensing.md