perf: lazy doc provider factory (#3330)

Co-authored-by: Alex Yang <himself65@outlook.com>

packages/y-provider/README.md

@@ -0,0 +1,8 @@
+# A set of provider utilities for Yjs
+
+## createLazyProvider
+
+A factory function to create a lazy provider. It will not download the document from the provider until the first time a document is loaded at the parent doc.
+
+To use it, first define a `DatasourceDocAdapter`.
+Then, create a `LazyProvider` with `createLazyProvider(rootDoc, datasource)`.