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

Docs: Merge EE doc/​customization to CE

See merge request gitlab-org/gitlab-ce!27604

doc/customization/issue_and_merge_request_template.md

+---
+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](https://docs.gitlab.com/ee/user/project/description_templates.html#setting-a-default-template-for-issues-and-merge-requests--starter).