Backport PR pandas-dev#44531: Add json normalize back into doc and move read_json to pd namespace

Author: phofl

doc/source/reference/io.rst

@@ -57,15 +57,18 @@ Excel
 
    ExcelWriter
 
-.. currentmodule:: pandas.io.json
+.. currentmodule:: pandas
 
 JSON
 ~~~~
 .. autosummary::
    :toctree: api/
 
    read_json
-   to_json
+   json_normalize
+   DataFrame.to_json
+
+.. currentmodule:: pandas.io.json
 
 .. autosummary::
    :toctree: api/