Overtype is a neat markdown editor that's as easy as jquery when it comes to "just adding it to a site".

The "magic trick" is that this is really just clever CSS on top of a textarea! You just define a <div class="editor> somewhere and then a new Overtype(".editor") will take care of the rest from JS. It also means that you can do stuff like:

let elem = document.querySelector(".editor textarea")
elem.addEventListener("input", e => console.log(elem.value))

Everything that is typed into the editor will now also appear in the console. You can check it for yourself!