Add page about GNOME online account integration
The GNOME online accounts integration is planned to be removed with GNOME 46 ( gnome-initial-setup#202 (closed)). Therefore the user should be informed in all apps using the online account integration that online accounts need to be setup in GNOME settings (targeted in gnome-calendar#1144 and gnome-contacts#331).
Additionally @wjt suggested to add a page in GNOME tours to inform the user about the GNOME online accounts integration with core apps and guide the user to link online accounts in settings.