fix: Add workaround to fix nokogiri installation error

After installing Nokogiri v1.14, the installation of Nokogiri v1.11 fails.

```
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/gems/2.7.0/gems/nokogiri-1.11.3/ext/nokogiri
/opt/hostedtoolcache/Ruby/2.7.8/x64/bin/ruby -I
/opt/hostedtoolcache/Ruby/2.7.8/x64/lib/ruby/2.7.0 -r
./siteconf20231010-5219-1a02hdd.rb extconf.rb
checking for whether -g is accepted as CFLAGS... yes
checking for whether -Winline is accepted as CFLAGS... yes
checking for whether -Wmissing-noreturn is accepted as CFLAGS... yes
Building nokogiri using packaged libraries.
Static linking is enabled.
Cross build is disabled.
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.
```
Therefore, install nokogiri from old versions by using `reverse_each`.

Author: toshimaru

Rakefile

@@ -20,7 +20,8 @@ end
 
 desc 'Generate adn build documentation for older versions of Rails'
 task :build_multi do
-  config['rails_versions'].each do |version, detail|
+  # WORKAROUND: use `reverse_each` instead of `each` to avoid nokogiri installation error
+  config['rails_versions'].reverse_each do |version, detail|
     dir = "#{SOURCE_DIR}/#{version}"
     mkdir dir unless Dir.exist?(dir)