As mentioned in T186450#5756473, former "wiki" files in the docs directory of the Wikibase code repository have been switched to Markdown files, and are compiled to HTML pages published on https://doc.wikimedia.org/Wikibase/master/php/
mediawiki.org pages that have previously linked to .wiki files should now be adjusted to point to the right pages.
Incomplete list of pages to adjust:
[] https://www.mediawiki.org/wiki/Extension:Wikibase_Client - links to docs directory and old hook docs
[] https://www.mediawiki.org/wiki/Extension:Wikibase_Repository - links to hook doc files
[] https://www.mediawiki.org/wiki/Extension:Wikibase_Client/Lua - links to lua.md file, could link to the HTML page?
That might be all according to https://www.mediawiki.org/w/index.php?title=Special:Search&limit=500&search=insource%3A%22phab%3Adiffusion%2FEWBA%2Fbrowse%2Fmaster%2Fdocs%22