Added sphinx-icalendar

[niccokunzmann]

I just created the sphinx-icalendar extension:

https://sphinx-icalendar.readthedocs.io/

I got excited. Have a look!

We could use it here, in order to highlight the code blocks.

[stevepiercy]

Other than going back in time in the change log, it looks great!

Would you like to create an issue for implementation by first-timers, following an example you set?

[niccokunzmann]

Awr! Right, I should have fixed the changelog four years ago, before I started!