Release instructions for jupyter-sphinx

Jupyter Sphinx uses a GitHub action to automatically push a new release to PyPI when a GitHub release is added.

To cut a new Jupyter Sphinx release, follow these steps:

Ensure that all tests are passing on master.
In _version.py, update the version number:
```
__version__ = "0.2.3"
```

Make a release commit and push to main

git add jupyter_sphinx/_version.py
git commit -m "RLS: 0.2.3"
git push upstream main

Create a new github release. The target should be main, the tag and the title should be the version number, e.g. v0.2.3.
Creating the release in GitHub will push a tag commit to the repository, which will trigger a GitHub action to build jupyter-sphinx and push the new version to PyPI. Confirm that the version has been bumped.
That's it!