Reason: https://github.com/wikimedia/mediawiki-extensions-CategorySlideShow README says "it's not really done yet" and was started 5 years ago. Unless more work is planned, I suggest we archive the extension.
- Archive documentation on mediawiki.org [there was none]
- Remove from mediawiki/extensions (along with in .gitmodules)
- Remove tests in integration/config (zuul/layout.yaml)
- Empty repository and add a OBSOLETE file.
- Archive the extension's diffusion repository (rECAS)
- Mark the repository read-only in Gerrit.
- Delete the GitHub mirror.