Add NextCloud or WebDAV sync options

Syncing notes is a huge task. Thousands of lines of code to get it right. But I'll consider it in the future if this becomes a blocker.

Company policy does not allow to store certain information on public clouds. So I store the data local in a nextcloud synced folder to share between 2 Macs. This works perfect. However, I can't use NotePlan for iOS, since syncing is only possible with iCloud (two variants).