**Reason**:
mediawiki/skins/CustomPage seems to have been a one off experiment by @tosfos and hasn't received any code since 2016. It also lack ressource files such as screen.less ( T154803 ).
- [ ] Archive documentation on `mediawiki.org` ( https://www.mediawiki.org/wiki/Skin:CustomPage )
- [ ] Remove from `mediawiki/extensions` / `mediawiki/skins` ( along with in `.gitmodules`)
- [X] Remove tests in `integration/config` ( `zuul/layout.yaml` )
- [ ] Remove from `translatewiki` ( `groups/MediaWiki/mediawiki-extensions.txt` / `groups/MediaWiki/mediawiki-skins.txt` )
- [ ] Empty repository and add a OBSOLETE file.
- [ ] Archive the extension's diffusion repository: ( https://phabricator.wikimedia.org/diffusion/ECSP/ )
- [ ] Mark the repository read-only in Gerrit: ( https://gerrit.wikimedia.org/r/#/admin/projects/mediawiki/skins/CustomPage )
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-skins-CustomPage