nixos/nextcloud: Add options services.nextcloud.autoUpdateApps

nixos/nextcloud: Add documentation for nextcloud app installation and updates

nixos/nextcloud: Enable autoUpdateApps in nextcloud test

nixos/nextcloud: Fix typo in nixos/modules/services/web-apps/nextcloud.xml

Co-Authored-By: Florian Klink <flokli@flokli.de>

nixos/nextcloud: Escape html in option description

nixos/nextcloud: Fix autoUpdateApps URL in documentation.

Co-Authored-By: Florian Klink <flokli@flokli.de>