Add a file in sync target to warn user that they shouldn't open any file

[laurent22]

Opening any file, even without changing it, on a Nextcloud sync target is going to corrupt it (See #9887).

To try to prevent the user from doing this we would like to add a file that appears at the top of the list, named something very visible such _⚠️_IMPORTANT_READ_FIRST_⚠️_.md. It would be automatically created when the sync target is initialised and should also be added to existing sync targets. It contains some information as to why these files should not be opened.

Edit: The difficulty with implementing this is that we don't want to have a check if the file exist on each sync. Traditionally the way to add a file to a sync target is to do a sync target migration but due to the complexity of it we avoid doing that now. Maybe a way would to be to check for example once a day if the file is there, and add it if it's not.