docs(changelog): add changelog entry for appearance API endpoint

changelogs/unreleased/feat-appearance-api.yml

+---
+title: Implement application appearance API endpoint
+merge_request: 20674
+author: Fabio Huser
+type: added

doc/api/appearance.md

 # Appearance API
 
-> [Introduced](https://gitlab.com/gitlab-org/gitlab/merge_requests/???) in GitLab 12.6.
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/merge_requests/20674) in GitLab 12.6.
 
 These API calls allow you to read and modify the GitLab appearance as visible in
 `/admin/appearance`. You have to be an administrator in order to perform this action.