add collaborative editing guide

docs/src/demos/Examples/CollaborativeEditingWs/server.js

@@ -0,0 +1,14 @@
+/*
+import { Server } from '@hocuspocus/server'
+import { LevelDB } from '@hocuspocus/leveldb'
+
+const server = Server.configure({
+  port: 1234,
+
+  persistence: new LevelDB({
+    path: './database',
+  }),
+})
+
+server.listen()
+*/