Create GitHub action to update calendar when new entry is added

[Kludex]

• Closes #89

Hi 👋

This PR aims to update a calendar whenever a PR is merged (if the file <current_year>.csv is modified).

I'll need help from the maintainers here. This is needed:

• Create a service on Google Cloud.
• Enable Google Calendar API.
• Modify the hard coded CALENDAR_ID by "primary", and use the email on the Service Accounts, see image below.

Missing

• [ ] Create the GitHub workflow.
• [ ] Add the credentials.json - I think there's a GitHub action to do something like this... I'm not sure yet.
• [ ] Need help from maintainers to do the administrative steps mentioned above.

Note: This already works locally. You can check the generated calendar: https://calendar.google.com/calendar/embed?src=0ji9k94abd2sehteqqqt4jo47c%40group.calendar.google.com&ctz=Europe%2FParis - I was debugging, so you'll see repeated events. :)