Document Dataset.load_async and DataArray.load_async (#11311)

Add both methods to the autosummary blocks in doc/api/io.rst so Sphinx
generates their reference pages. Without these entries no pages are
built, and the :py:meth: cross-references to them in whats-new.rst
render as plain text.

Co-authored-by: Claude <noreply@anthropic.com>

Author: TomNicholas

doc/api/io.rst

@@ -30,6 +30,7 @@ Dataset methods
    Dataset.filter_by_attrs
    Dataset.info
    Dataset.load
+   Dataset.load_async
    Dataset.persist
    Dataset.unify_chunks
 
@@ -62,6 +63,7 @@ DataArray methods
    DataArray.compute
    DataArray.persist
    DataArray.load
+   DataArray.load_async
    DataArray.unify_chunks
 
 DataTree methods