chore(grunt): do not rewrite gif urls
docs(guide/forms): add ✨ animated gifs ✨
chore(docs): copy the correct docs assets

The docs images had been duplicated in

```
docs/img/
```

and

```
docs/app/assets/img
```

This commit fixes the gulp build to use the doc images from `docs/img` and
removes the duplocates from `docs/app/assets/img`

Closes #9655