Automated publishing of docs to `gh-pages` branch

At the moment I have to do the following:

```sh
pip install sphinx
just docs
cd docs/_build/html
git init
git remote add origin git@github.com:rossmacarthur/serde.git
git fetch
git reset origin/gh-pages
git add .
git commit -m "publish docs for $(git -C ../../.. rev-parse --short HEAD)"
git push origin HEAD:gh-pages
```