**Reason**: BookManagerv2 has been archived recently, and BookManager has a big warning at the top of https://www.mediawiki.org/wiki/Extension:BookManager saying that this is no longer mantained and that we look for the (now also archived v2 version). As such, we should archive this one too.
- [x] Archive documentation on `mediawiki.org` https://www.mediawiki.org/wiki/Extension:BookManager
- [ ] Remove from `mediawiki/extensions` (along with in `.gitmodules`)
- [ ] Remove tests in `integration/config` (`zuul/layout.yaml`)
- [ ] Remove from `translatewiki` (`groups/MediaWiki/mediawiki-extensions.txt`)
- [ ] Empty repository and add a OBSOLETE file.
- [x] Archive Phabricator project #mediawiki-extensions-bookmanager
- [x] Edit Phabricator project #mediawiki-extensions-bookmanager description for associated extension/skin with a link to this ticket
- [ ] Archive the extension's diffusion repository rEBMA
- [ ] Mark the repository read-only in Gerrit
- [ ] Delete the GitHub mirror: https://github.com/wikimedia/mediawiki-extensions-BookManager