There is no <code>core</code> directory under mediawiki. All the extension etc. doc pages start with <code>cd mediawiki/core</code>. Was this changed at some point and docs not updated?
Topic on Talk:MediaWiki-Docker
You're right, the main page says clone to "mediawiki" and then each extension page says go to mediawiki/core.
The extension pages should probably say "cd /path/to/where/you/cloned/mediawiki" or just say, "make sure you are in the MediaWiki core directory before proceeding".
I updated one (Echo) from where I forked Translate.
Apologies for the late reply. I took a look a the majority of the pages. (I'm still working on the last few.) 41% of the pages I've looked seem to be correct. (My assumption is that if repository's Selenium test suite passes, the extension/skin is installed correctly.) See T256239. I'll update the pages so they follow a better template. See MediaWiki-Docker/Extension/AbuseFilter for an example. Please let me know if you have any feedback or questions. Here or in Phabricator.