fix: Add csv for Ruby v2.4

```
bundler: failed to load command: jekyll (/home/runner/work/railsdoc.github.io/railsdoc.github.io/vendor/bundle/ruby/3.4.0/bin/jekyll)
<internal:/opt/hostedtoolcache/Ruby/3.4.6/x64/lib/ruby/3.4.0/rubygems/core_ext/kernel_require.rb>:37:in 'Kernel#require': cannot load such file -- csv (LoadError)
```

Author: toshimaru

Gemfile

@@ -1,13 +1,15 @@
 source 'https://rubygems.org'
 
 gem 'jekyll'
+
+gem 'csv'
 gem 'nokogiri', '1.15.6'
 gem 'rake'
 gem 'webrick'
 
 group :jekyll_plugins do
-  gem 'github-pages'
   gem 'faraday', '~> 2.8.1' # For Ruby v2.x
+  gem 'github-pages'
   gem 'jekyll-include-cache'
   gem 'jekyll-toc'
 end