Set HTML writer version in conf.py to workaround sphinx whitespace issue.

* Implement improved Softplus

* Extend gradient test to wider range

* Add numerical accuracy test

* Fix indentation

This change prevents Python build changes (e.g. CPython -> PyPY) during
dependency installations.

They just wrapped around numpy.seterr, which users should call directly instead.
This also changes the default behaviour from setting numpy.seterr(all="ignore") to leaving all at its numpy default.

* Add how to add JAX Ops rst doc

* Update doc/JaxOps.rst
Co-authored-by: Thomas Wiecki <thomas.wiecki@gmail.com>
Co-authored-by: Thomas Wiecki <thomas.wiecki@gmail.com>

Closes #55

Closes #203

And specify their purpose and public API.

Closes #23.