DOC: `pandas.json_normalize` missing from API Reference portion of documentation
#### Location of the documentation

https://pandas.pydata.org/docs/dev/reference/api/pandas.json_normalize.html (currently responds with a `404 Not Found`)

#### Documentation problem

While the source code does contain API reference documentation for `pandas.json_normalize` (previously `pandas.io.json.json_normalize`) as seen in [`/pandas/io/json/_normalize.py`](https://github.com/pandas-dev/pandas/blob/master/pandas/io/json/_normalize.py#L240-L381), it isn't available in the documentation for the current version of pandas hosted on `pandas.pydata.org`.

#### Suggested fix for documentation

At least one older version of `pandas` _does_ have documentation that includes a `json_normalize` API reference under its old name ([`pandas.io.json.json_normalize` in v0.25.0](https://pandas.pydata.org/pandas-docs/version/0.25.0/reference/api/pandas.io.json.json_normalize.html)), so it seems feasible that it was removed by accident when its old name was [deprecated in v1.0.0](https://github.com/pandas-dev/pandas/blob/master/pandas/io/json/_normalize.py#L524-L526). Attempting to visit [`pandas.io.json.json_normalize` in the current version of the documentation](https://pandas.pydata.org/docs/dev/reference/api/pandas.io.json.json_normalize.html) redirects to the new name (`pandas.json_normalize`), which responds with a 404.
