fix: docs/Gemfile & docs/Gemfile.lock to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-REXML-6861566

Author: snyk-bot

docs/Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem 'jekyll', '3.9.0'
+gem 'jekyll', '3.9.1'
 
 group :jekyll_plugins do
   gem 'jekyll-paginate'

docs/Gemfile.lock

@@ -1,20 +1,20 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.8.0)
-      public_suffix (>= 2.0.2, < 5.0)
+    addressable (2.8.6)
+      public_suffix (>= 2.0.2, < 6.0)
     colorator (1.1.0)
-    concurrent-ruby (1.1.7)
-    em-websocket (0.5.1)
+    concurrent-ruby (1.2.3)
+    em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
-      http_parser.rb (~> 0.6.0)
+      http_parser.rb (~> 0)
     eventmachine (1.2.7)
-    ffi (1.13.1)
+    ffi (1.16.3)
     forwardable-extended (2.6.0)
-    http_parser.rb (0.6.0)
+    http_parser.rb (0.8.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
-    jekyll (3.9.0)
+    jekyll (3.9.1)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
@@ -33,34 +33,36 @@ GEM
     jekyll-watch (2.2.1)
       listen (~> 3.0)
     json (2.3.1)
-    kramdown (2.3.1)
+    kramdown (2.4.0)
       rexml
-    liquid (4.0.3)
-    listen (3.2.1)
+    liquid (4.0.4)
+    listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (4.0.6)
-    rb-fsevent (0.10.4)
+    public_suffix (5.0.5)
+    rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
     redcarpet (3.5.1)
-    rexml (3.2.5)
-    rouge (3.22.0)
+    rexml (3.2.8)
+      strscan (>= 3.0.9)
+    rouge (3.30.0)
     safe_yaml (1.0.5)
     sass (3.7.4)
       sass-listen (~> 4.0.0)
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+    strscan (3.1.0)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
-  jekyll (= 3.9.0)
+  jekyll (= 3.9.1)
   jekyll-paginate
   json
   redcarpet