Retire the grunt-based build

Right now, Git for Windows' home page is built using `grunt`, and the
result is then committed and uploaded to GitHub Pages.

However, a much more natural development experience can be achieved by
using Hugo (https://gohugo.io/) instead.

In preparation for migrating to Hugo, let's drop all `grunt`-based
stuff.

Note that the `create-screenshot-thumbnails.js` script would still need
the `jimp` dependency defined in `package.json`, but that script is not
run on a regular basis.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho