docs: add webrick dependency

webrick needs to be installed for ruby versions >3, which should ease
local testing for someone running newer versions. This is compatible
with running on ruby 2.7.4, the version used on github pages.

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/Gemfile b/docs/Gemfile
index fd9a462..b88481b 100644
--- a/docs/Gemfile
+++ b/docs/Gemfile
@@ -22,3 +22,5 @@
 gem 'tzinfo-data', platforms: [:mingw, :mswin, :x64_mingw, :jruby]
 
 gem "jemoji", "~> 0.12.0"
+
+gem "webrick", "~> 1.8"
diff --git a/docs/Gemfile.lock b/docs/Gemfile.lock
index 58b2529..4335856 100644
--- a/docs/Gemfile.lock
+++ b/docs/Gemfile.lock
@@ -249,6 +249,7 @@
       unf_ext
     unf_ext (0.0.8.2)
     unicode-display_width (1.8.0)
+    webrick (1.8.1)
 
 PLATFORMS
   ruby
@@ -257,6 +258,7 @@
   github-pages
   jemoji (~> 0.12.0)
   tzinfo-data
+  webrick (~> 1.8)
 
 BUNDLED WITH
    1.17.2