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 FrameworkThey have a great getting-started guide for collaborative editing with Yjs & Milkdown:
Getting Started Guide
Plugin Based WYSIWYG Markdown Editor Framework