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

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-6228056

docs/Gemfile

@@ -16,4 +16,4 @@ gem 'json'
 gem 'rb-fsevent'
 
 # For markdown header cleanup
-gem 'sanitize', '~>2.0'
+gem 'sanitize', '~> 2.1', '>= 2.1.0'

docs/Gemfile.lock

@@ -47,22 +47,24 @@ GEM
       rb-fsevent (>= 0.9.3)
       rb-inotify (>= 0.9)
     mercenary (0.3.4)
-    mini_portile (0.6.0)
-    nokogiri (1.6.3.1)
-      mini_portile (= 0.6.0)
+    mini_portile2 (2.8.5)
+    nokogiri (1.15.6)
+      mini_portile2 (~> 2.8.2)
+      racc (~> 1.4)
     parslet (1.5.0)
       blankslate (~> 2.0)
     posix-spawn (0.3.9)
     pygments.rb (0.6.0)
       posix-spawn (~> 0.3.6)
       yajl-ruby (~> 1.1.0)
+    racc (1.7.3)
     rake (10.3.2)
     rb-fsevent (0.9.4)
     rb-inotify (0.9.5)
       ffi (>= 0.5.0)
     redcarpet (3.1.2)
     safe_yaml (1.0.4)
-    sanitize (2.0.6)
+    sanitize (2.1.1)
       nokogiri (>= 1.4.4)
     sass (3.3.14)
     timers (1.1.0)
@@ -79,4 +81,7 @@ DEPENDENCIES
   json
   rake
   rb-fsevent
-  sanitize (~> 2.0)
+  sanitize (~> 2.1, >= 2.1.0)
+
+BUNDLED WITH
+   2.1.4