Update gems and add .ruby-version for rbenv usage

Fixes vulnerabilities in gems.

Had to add .ruby-version for `bundle install` to work. Used the same
ruby version as specified in https://pages.github.com/versions.json,
which is linked from Gemfile.

docs/Gemfile.lock

@@ -1,34 +1,45 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (7.0.4.3)
+    activesupport (7.1.1)
+      base64
+      bigdecimal
       concurrent-ruby (~> 1.0, >= 1.0.2)
+      connection_pool (>= 2.2.5)
+      drb
       i18n (>= 1.6, < 2)
       minitest (>= 5.1)
+      mutex_m
       tzinfo (~> 2.0)
-    addressable (2.8.4)
+    addressable (2.8.5)
       public_suffix (>= 2.0.2, < 6.0)
+    base64 (0.1.1)
+    bigdecimal (3.1.4)
     coffee-script (2.4.1)
       coffee-script-source
       execjs
     coffee-script-source (1.11.1)
     colorator (1.1.0)
-    commonmarker (0.23.9)
+    commonmarker (0.23.10)
     concurrent-ruby (1.2.2)
+    connection_pool (2.4.1)
     dnsruby (1.70.0)
       simpleidn (~> 0.2.1)
+    drb (2.1.1)
+      ruby2_keywords
     em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0)
     ethon (0.16.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
-    execjs (2.8.1)
-    faraday (2.7.4)
+    execjs (2.9.1)
+    faraday (2.7.11)
+      base64
       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 +97,7 @@ 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)
@@ -209,20 +220,23 @@ GEM
       jekyll (>= 3.5, < 5.0)
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
-    minitest (5.18.0)
-    nokogiri (1.14.3-x86_64-darwin)
+    minitest (5.20.0)
+    mutex_m (0.1.2)
+    nokogiri (1.15.4-arm64-darwin)
+      racc (~> 1.4)
+    nokogiri (1.15.4-x86_64-darwin)
       racc (~> 1.4)
     octokit (4.25.1)
       faraday (>= 1, < 3)
       sawyer (~> 0.9)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.7)
-    racc (1.6.2)
+    racc (1.7.1)
     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)
@@ -249,6 +263,7 @@ GEM
     unicode-display_width (1.8.0)
 
 PLATFORMS
+  arm64-darwin-21
   x86_64-darwin-21
 
 DEPENDENCIES