Add hint to the starting introduction to select storage place (and grant universal acces rights)

[JensKorte]

After installing kiwix-universal-2023-11-10.apk I went to the preferences and changed the storage place to external and granted the access storage permissions. For new users this might be a nice detail to know about.

The starting introduction consists of 2 pages. How about adding a 3. page with hints about the usage of external storage/sd card is selectable in the preferences. And for the non-google versions (I think): the universal storage access rights can be activated in the preferences.