"Add to calendar" button for tournaments and more

[superuser-does]

Proposal

It would be fantastic if you could have an "Add to calendar" button to easily add an upcoming tournament or event to the calendar tool of your choice. This would be especially useful for scheduled lichess tournaments of some significance, like Monthlies, Marathons, Titled Arenas, Anniversaries, and so on.

If someone registers for a tournament in advance, or if they simply want a reminder on their computer or phone near when it starts, they could get a lot of value out of such a feature. It should be noted that calendar apps generally handle the reminder part - Lichess should only put in the specific time the event commences.

The calendar event would in turn sync across all their devices like Apple/Android by default, and notify them when it's about to start, increasing engagement. Furthermore, Windows 10, Mac as well as the major Linux DEs (GNOME, KDE) all integrate with web calendars and encourage you to hook these in on install - giving a desktop notification in case our Lichess player doesn't have their phone nearby.

There would be no cost to Lichess because the calendar app/service takes care of notifying the user. We simply need a way for them to easily create an event in their calendar, should they so wish.

Requirements & implementation:

• Links needed for Google Calendar, Outlook Web and Yahoo Calendar. These are really just URLs with parameters. Trivial.
• iCalendar (.ics) file generator - this is the default & standard format for Outlook (desktop edition), Apple's Calendar, and Thunderbird's Lightning, among other offline email programs. It also works with apps like Google Calendar and iOS Calendar. Very few fields of what the iCalendar format supports are actually needed for Lichess's purpose, just a small subset will do.

There are third-party services you could call on, but it's not particularly challenging to have this in-house.

Documentation on adding these to your site is scant, but helpfully some people have compiled it all here, and I can confirm it's up to date. They even link to UI widget code you could use.