Nightly documentation builds

[Reinmar]

It'll be nice to have nightly documentation builds available under https://ckeditor5.github.io/docs/ while we work on the documentation.

Once we release 1.0.0 alpha and make https://ckeditor5.github.io/docs/ publicly available from https://ckeditor5.github.io we will need to move nightly builds to some other URL (e.g. https://ckeditor5.github.io/docs/nightly) because under the official URL we should have documentation build together with a release. Hence, we can also start with this URL already.

I imagine that this task will be performed by Travis's cron job once every couple of hours. We can use the same mechanism with GH token which we use for revisions. Assuming that ckeditor5 repo is cloned the process should look more or less like this:

git clone https://github.com/CKEditor5/ckeditor5.github.io.git
npm run install-optional-dependencies
gulp docs
rm -rf ckeditor5.github.io/docs/nightly/*
cp -R build/docs/* ckeditor5.github.io/docs/nightly

cd ckeditor5.github.io
echo "https://${ process.env.GITHUB_TOKEN }:@github.com" > .git/credentials 2> /dev/null
git config credential.helper "store --file=.git/credentials"

git commit -a -m "Documentation build."
git push origin master

We should also add the entire docs directory to ignored paths in robots.txt because we don't want to have this documentation crawled. It will be available under a different URL once we go with the final release and it's not worth dealing with some redirects from GH pages.

[Reinmar]

Note: Due to how Umberto builds the docs, the actual path to docs index will be: https://ckeditor5.github.io/docs/nightly/ckeditor5/latest.