Add an event section to the website.

This is meant both as a way to get an overview of upcoming FreeBSD
related events, and as a way to structure links to pages with
information / pictures / etc. from past events.

The page automatically determines which events are upcoming and which
are past, by comparing the event dates with the current date.

Discussed on: -www
Inspired by PR: www/53674
Which was submitted by: Josef El-Rayes <j.el-rayes@daemon.li>