A good free online resource is Yaron Koren's [https://www.workingwithmediawiki.com/book/ Working with MediaWiki]. Yaron Koren is a contributor to the MediaWiki software community, particularly around integrating data and journalism (he's the primary author of the Cargo extension).
There's also a simple [[Wiki Tutorial]] made over the years for the course.
* [https://www.workingwithmediawiki.com/book/chapter4.html Wikitext] : a "lightweight markup", Headers, etc. (there are also some useful [https://www.workingwithmediawiki.com/book/chapter4.html#toc-Section-31 Behaviour Switches] like <nowiki>__NOTOC__</nowiki>)
* Section editing (good way to avoid edit conflicts)