Rather frustratingly the docs mention one:
https://docs.nextcloud.com/server/19/user_manual/pim/calendar.html?highlight=embedding%20code
and the web is plastered with references to one:
not to mention loads of articles looking at how to fix the refusal to load it due to cross site security settings.
But the doc does not tell me where to get one, and UI does not either. How can I get a calendar embedding code?
