Add webpage for usages

[niccokunzmann]

It would be nice to see how people are using the OpenWebCalendar.
There is an examples page but that is different from seeing other embeds of the calendar.
It would be nice to have a page that shows examples of how people use the calendar.

See also:

• https://open-web-calendar.quelltext.eu/templates/
• links not shown in description #388 (comment)

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