Add json normalize back into doc and move read_json to pd namespace
- [x] closes #42540 

read_json looked like it had to be used like pandas.io.json.read_json and json_normalize was missing alltogether. Maybe backporting as well?

Edit: Build Docs locally, look fine now
thanks @phofl  @meeseeksdev backport 1.3.x Something went wrong ... Please have a look at my logs.