Fix styleguide link
One-sentence description
The styleguide link on the website is broken.
Current behaviour / Reproducing the bug
Try to download the styleguide, get a nextcloud error page.
Expected behaviour
The link downloads a zip with the styleguide.
I know that sharing a folder from a shared drive in GSuite is not possible, but the link has been broken for a while and will be even more broken when NextCloud goes offline.