Import system for deck data

[juliusknorr]

Export is currently possible as a custom json file through occ. It should be possible to import such files through the import handling implemented in #3182 by adding an implementation to properly handle the json format.

[benjamindahl]

definetely needed

[juliusknorr]

Done as occ command in #4939

[alexanderdd]

starting with NC 32.0.0.9 it is also possible to export/import via GUI, but not in user docs yet https://deck.readthedocs.io/en/latest/User_documentation_en/

occ export/import is documented here https://github.com/nextcloud/deck/blob/main/docs/export-import.md