Warn about datadirectory changing during installation

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Is your feature request related to a problem? Please describe.
There's no supported way to change the datadirectory and people who do it usually encounter issues. There's a warning in the admin manual saying "This parameter should not be changed after finishing the initial installation of Nextcloud. Doing so might lead to side effects and affect several features". Some users miss this warning.

Describe the solution you'd like
If we don't want that to reproduce, IMO, we should warn admins as early as possible during the installation process, that this is not a recommended approach to keep /data inside /nextcloud, AND that changing afterward is not supported. So they take action before setup, and they don't have to change it later on.

Start by reading the admin manual's datadirectory guidance and tracing the installation process where the data directory is configured. Add an early warning that keeping /data inside /nextcloud is not recommended and that changing the directory after installation is unsupported. Confirm that the warning appears during setup.