DOC: #7828 Add json_normalize to api docs.
closes #7828 

Is this all we need?

yep
 thanks!
 @lexual Can you put the `read_json` and `json_normalize` in two different `autosummary` directives? As I want that `read_json` is still displayed as being top level pandas pd.read_json (instead of pd.io.json.read_json)
