DOC: how-to-io.rst: document solution for NumPy JSON serialization

Users have reported that NumPy types are not JSON serializable. This is documented, but no solution is recommended. This PR suggests trying a custom NumPy JSON encoder, several of which are easily found online. Whether NumPy should provide a solution as part of the package remains an open feature request.

[skip actions] [skip azp] [skip cirrus]

doc/source/user/how-to-io.rst

@@ -303,8 +303,10 @@ NetCDF (see :ref:`how-to-io-large-arrays`).
 Write or read a JSON file
 =========================
 
-NumPy arrays are **not** directly
+NumPy arrays and most NumPy scalars are **not** directly
 `JSON serializable <https://github.com/numpy/numpy/issues/12481>`_.
+Instead, use a custom `json.JSONEncoder` for NumPy types, which can
+be found using your favorite search engine.
 
 
 .. _how-to-io-pickle-file: