Merge branch 'docs-repo-merge-1-customization-ee' into 'master'

Docs: (EE Port) Merge EE doc/​customization to CE

See merge request gitlab-org/gitlab-ee!11450

doc/customization/issue_and_merge_request_template.md

 ---
-redirect_to: '../user/project/description_templates.md#setting-a-default-template-for-issues-and-merge-requests--starter'
+redirect_to: 'https://docs.gitlab.com/ee/user/project/description_templates.html#setting-a-default-template-for-issues-and-merge-requests--starter'
 ---
 
-This document was moved to [description_templates](../user/project/description_templates.md#setting-a-default-template-for-issues-and-merge-requests--starter).
+This document was moved to [description_templates](https://docs.gitlab.com/ee/user/project/description_templates.html#setting-a-default-template-for-issues-and-merge-requests--starter).