Convert Mediawiki to Markdown (MediaWiki to GFM)

I am using Mediawiki with the "Wikitext" syntax for many years, but now I'm at a point where I want to switch to the "Markdown" syntax.
Here I show you how you can "simply" switch your Mediawiki sites to Markdown files.

First thanks @PhilipAshlock and @OutOfControl for the nice GitHub project: https://github.com/outofcontrol/mediawiki-to-gfm