add to doc page: caldav + 2FA workaround on iOS

[humptydumpty]

If 2FA is enabled for the Nextcloud user, that user cannot use their password to connect to caldav to sync calendars on iOS. The solution is to create an app password (see https://help.nextcloud.com/t/2fa-in-combination-with-card-caldav-sync/3467/4).

In your Nextcloud, browse to:
personal settings > security > devices & sessions > enter an app name and click the button "Create new app password".

This might not be iOS specific. There is no mention on how to set up Caldav in the docs. Perhaps a note on this can also be added. Thanks!