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

Nextcloud has shown to be incompatible with Intel UHD 620 graphics cards as discussed [here](https://github.com/nextcloud/desktop/issues/896#issuecomment-727836778).

Please make this clear in the download section and/or in the description in the [system requirements](https://docs.nextcloud.com/server/latest/admin_manual/installation/system_requirements.html).

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

Review the linked compatibility discussion and the system requirements page, then locate the relevant download or requirements content. Confirm how the Intel UHD 620 limitation should be described and add a clear warning in the appropriate location; done means users can see the incompatibility before deployment.

Written by the indexing model from the issue text.