Configure analytics on `plausible.scala-lang.org`

[hamzaremmal]

We should add the following snippet to all the html code under workshop.scala-lang.org:

<script defer data-domain="workshop.scala-lang.org" src="https://plausible.scala-lang.org/js/plausible.js"></script>

[hamzaremmal]

Configuration on plausible.scala-lang.org has been already done by me earlier.