Gemfile: Add webrick dependency

So far, what stopped ruby 3.0 working with jekyll was an unmerged PR in
the pathutil package[1].  The maintainer being inactive, jekyll chose to
instead remove this dependency[2] and GitHub pages has also upgraded its
jekyll version to something recent enough now[3].

All that's left to do for us is to add webrick[4] as a dependency to
the Gemfile.

Fixes: #37

[1]: envygeeks/pathutil#5
[2]: jekyll/jekyll@55e3648
[3]: github/pages-gem#833
[4]: jekyll/jekyll#8523

Author: slotThe

Gemfile

@@ -1,3 +1,4 @@
 source "https://rubygems.org"
 
 gem "github-pages", group: :jekyll_plugins
+gem "webrick", "~> 1.7"