Add clear hint or warning that Nextcloud is incompatible with Intel UHD 620 graphics cards

[dossma]

Nextcloud has shown to be incompatible with Intel UHD 620 graphics cards as discussed here.

Please make this clear in the download section and/or in the description in the system requirements.

Otherwise, people may deploy Nextcloud only to find out after much work, that it is is incompatible.