Introduce events calendar

[Tomasz-Kluczkowski]

This one is for the future when we have the main site done but I think it would be nice to eventually have a calendar with the events. Jekyll does not have a backend but the .md files can perform the role of entities/fields etc. and provide necessary data.
Some inspiration of what can be done (random google search):
https://www.raymondcamden.com/2017/02/24/an-example-of-a-static-site-with-a-dynamic-calendar/

https://gist.github.com/iani/8906741