I started using MDN Web Docs by contributing to it. That surely is not the typical way, but because I'm not a web developer, I was not the typical reader/user.
I started contributing to open source through the Open Advice project run by the French association, Framasoft. I was lucky to discover great people and learned a ton along the way. Some folks were also members of the French-speaking Mozilla community, one thing leading to another, and I was soon on IRC and subscribed to a mailing list (pepperidge farm remembers). Thanks to all the goofy people out there. :)
My first steps as a Mozilla contributor were expanding the Glossary section of MDN Web Docs in French after a call for help on the said mailing list. I also contributed to other Mozilla projects but stuck with MDN for a while.