add "board export/import" via GUI to user docs, add "board export/import" via occ to admin docs

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starting with NC 32.0.0.9 it is also possible to export/import boards via GUI

IMO these steps need to be done:

• add how to export/import via GUI to https://github.com/nextcloud/deck/blob/main/docs/User_documentation_en.md#8-import-boards
• create admin docs section
• add admin docs link to app store
• add occ export/import to admin docs, see here https://github.com/nextcloud/deck/blob/main/docs/export-import.md
• delete https://github.com/nextcloud/deck/blob/main/docs/export-import.md

Question: Why some of the things in https://github.com/nextcloud/deck/tree/main/docs get displayed on https://deck.readthedocs.io/ (for example user documentation), but not all of them, for example https://github.com/nextcloud/deck/blob/main/docs/export-import.md ?

related: https://github.com/nextcloud/deck/issues/3507

Start with docs/User_documentation_en.md at the import-boards section and docs/export-import.md, then check how the repository’s docs appear on ReadTheDocs. Update the user and admin documentation for GUI and occ export/import, add the admin documentation link to the app store, and remove docs/export-import.md once its content is covered.