Add a build_for_local_ruby task to simplify local testing flow

A common way to test RDoc changes is to build `ruby/ruby`'s documentation
with the latest RDoc changes. When RDoc was a default gem, we can
sync it to `ruby/ruby` with its `tool/sync_default_gems.rb` script.

Now that RDoc is a bundled gem, we need to use a different method to
sync it to `ruby/ruby`. And so far building it and moving it to
`ruby/ruby`'s bundled gems folder is the easiest way to do it.

Rakefile

@@ -99,6 +99,11 @@ task :clean do
   end
 end
 
+desc "Build #{Bundler::GemHelper.gemspec.full_name} and move it to local ruby/ruby project's bundled gems folder"
+task build_for_local_ruby: :build do
+  sh "mv #{path}.gem ../ruby/gems/"
+end
+
 begin
   require 'rubocop/rake_task'
 rescue LoadError