chore: Update dependency jekyll to v4

docs/Gemfile.lock

@@ -6,7 +6,7 @@ GEM
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
       tzinfo (~> 2.0)
-    addressable (2.8.1)
+    addressable (2.8.5)
       public_suffix (>= 2.0.2, < 6.0)
     coffee-script (2.4.1)
       coffee-script-source
@@ -28,7 +28,7 @@ GEM
       faraday-net_http (>= 2.0, < 3.1)
       ruby2_keywords (>= 0.0.4)
     faraday-net_http (3.0.2)
-    ffi (1.15.5)
+    ffi (1.16.3)
     forwardable-extended (2.6.0)
     gemoji (3.0.1)
     github-pages (228)
@@ -86,7 +86,7 @@ GEM
       activesupport (>= 2)
       nokogiri (>= 1.4)
     http_parser.rb (0.8.0)
-    i18n (1.12.0)
+    i18n (1.14.1)
       concurrent-ruby (~> 1.0)
     jekyll (3.9.3)
       addressable (~> 2.4)
@@ -224,7 +224,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
+    rexml (3.2.6)
     rouge (3.26.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)