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, change the "release type" section to "final" e.g.:
```
version_info = (0, 2, 3, "final")
```

Make a release commit and push to master

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

Create a new github release. The target should be master, the tag and the title should be the version number, e.g. 0.2.3.
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.
In _version.py, bump the minor version and change the "release type" section to "alpha". make sure to include a number after the release type, e.g.:
```
version_info = (0, 2, 4, "alpha", 1)
```
That's it!

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