I created https://markdownshare.com/ for the same purpose; but due to abuse I've retired it.

Shame really, because I liked the service and it was quick/simple to build.

It's neat, but probably needs a bit more features for it to be more useful than pastebin.

Well, there's StackEdit [1], and we use it (at work, for an end-of-sprint automated list of finished stuff for the demo)

[1]: https://stackedit.io/

There is https://txti.es/ . Additionally, GitHub Gists will render markdown.