[NextCloud] Update doc, remove usuned files, add CeT theme

pica-nextcloud/13.0/Dockerfile

pica-nextcloud/13.0/config/apache-pretty-urls.config.php

pica-nextcloud/13.0/config/apcu.config.php

pica-nextcloud/13.0/config/apps.config.php

pica-nextcloud/13.0/config/autoconfig.php

pica-nextcloud/13.0/cron.sh

pica-nextcloud/13.0/entrypoint.sh

pica-nextcloud/15.0/Dockerfile

pica-nextcloud/15.0/config/apache-pretty-urls.config.php

pica-nextcloud/15.0/config/apcu.config.php

pica-nextcloud/15.0/config/apps.config.php

pica-nextcloud/15.0/config/autoconfig.php

pica-nextcloud/15.0/config/redis.config.php

pica-nextcloud/15.0/cron.sh

pica-nextcloud/15.0/entrypoint.sh

pica-nextcloud/15.0/upgrade.exclude

pica-nextcloud/README.md

pica-nextcloud/cet/README.md

pica-nextcloud/cet/docker-compose.yml

pica-nextcloud/cet/favicon.png