Added webrick to gemfile to enable page rendering with ruby 3

[bvreede]

This change was necessary on my machine to generate the server with Ruby 3, as webrick no longer comes bundled with Ruby. I have not confirmed that it does not break the workflow with earlier versions of Ruby, however.

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Closes #180

[bvreede]

Can confirm that it still works with an earlier version of Ruby (2.6).

[Peter9192]

Maybe we can also report this with https://github.com/carpentries/lesson-example and https://github.com/escience-academy/lesson-template, suggest this fix and see if they have any feedback/suggestions? I can imagine they have a better overview of how this should work than we do.