Milkdown

Milkdown is a ProseMirror based editor that integrates Yjs as the collaborative editing solution. It's a plugin driven WYSIWYG markdown editor.

Milkdown Homepage

Plugin Based WYSIWYG Markdown Editor Framework

They have a great getting-started guide for collaborative editing with Yjs & Milkdown:

Getting Started Guide

Plugin Based WYSIWYG Markdown Editor Framework