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

[laurent22]

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

[RadCod3]

Hi, I've been working on this and managed to figure out how to add a file to Nextcloud targets. However, I needed some clarifications.

Here is the change I made: RadCod3@59167b3

This modification enables the addition of a file to a Nextcloud target by checking if it has the warning file and adding it if it doesn't. I observed that when connecting a different Joplin client to the sync remote, the console shows the delta algorithm finding 4 new items. These items in the Nextcloud include the warning file, one for a note, one for a notebook (?), and the info.json file. The sync fetches all four of these files, but the warning file doesn't appear in the notes section of the Joplin app. I believe this is what we need, but I'm curious as to why it was filtered. Could it be related to the file not having a footer with id, parent_id, etc.?

Regarding what to add to the warning file, how does the following look?

THIS WAS CREATED AUTOMATICALLY BY JOPLIN. 

# **PLEASE DO NOT EDIT ANY FILES CREATED BY JOPLIN USING THE NEXTCLOUD TEXT EDITOR AS IT CORRUPTS THE FILES.**

FOR MORE INFORMATION ABOUT THIS ISSUE, PLEASE REFER TO THE FOLLOWING LINK: 
[ Nextcloud sync: Very easy to cause data corruption with Nextcloud's rich text editor #9887 ]
(https://github.com/laurent22/joplin/issues/9887)