Add CI that auto-deploys Sphinx docs after merges to master

[rgommers]

It should do something like (now doing locally)

# Install packages in requirements.txt
git clean -xdf
pushd spec
make html
popd
(git checkout --orphan tmp && git branch -D gh-pages || true)
git checkout --orphan gh-pages
git reset --hard
cp -R spec/_build/html/ latest
touch .nojekyll
git add .nojekyll
git add latest
git commit -m "Commit API Standard doc build";
git push --set-upstream origin gh-pages --force

Put the above in a file deploy-ghpages.sh at .. from the repo root, then run (from master, in the repo root):

source ../deploy-ghpages.sh