Bring back :doc task

which has been removed at 11bb81149f
but still in need for generating a complete set of haml.info docs

Rakefile

@@ -112,5 +112,30 @@ task bench: :compile do
   exit system(*cmd)
 end
 
+namespace :doc do
+  task :sass do
+    require 'sass'
+    Dir["yard/default/**/*.sass"].each do |sass|
+      File.open(sass.gsub(/sass$/, 'css'), 'w') do |f|
+        f.write(Sass::Engine.new(File.read(sass)).render)
+      end
+    end
+  end
+
+  desc "List all undocumented methods and classes."
+  task :undocumented do
+    command = 'yard --list --query '
+    command << '"object.docstring.blank? && '
+    command << '!(object.type == :method && object.is_alias?)"'
+    sh command
+  end
+end
+
+desc "Generate documentation"
+task(:doc => 'doc:sass') {sh "yard"}
+
+desc "Generate documentation incrementally"
+task(:redoc) {sh "yard -c"}
+
 task default: %w[compile hamlit:test]
 task test: %w[compile test:all]