Add a .gitignore file, and remove _site from git

This PR just adds in a `.gitignore` file (as per the [Jekyll gitignore template](https://github.com/github/gitignore/blob/master/Jekyll.gitignore)) so that the `_site` folder is ignored in the future, and also removes the files in that output folder which had already been committed to git in the past. This is handy because it means that you won’t see the same change reflected in two files (the source, and the output) in each diff from now on. If you have any questions about this, just ask and I’ll do what I can to clarify :grinning: 

/cc @fehler @timpotter @PixelPaper