docs: annotate Ruby configuration and plugin APIs with YARD types

examples/plugins/greeting.rb

@@ -1,11 +1,16 @@
 Vamp.plugin "greeting", requires: ["text"]
 
+# @param editor [Vamp::Editor]
+# @param args [String]
 Vamp.command "greet" do |editor, args|
+  # @type [String]
   name = args.empty? ? "world" : args
   editor.insert "Hello, #{name}!\n"
   editor.message "Greeting inserted by Ruby"
 end
 
+# @param editor [Vamp::Editor]
+# @param _args [String]
 Vamp.command "upper" do |editor, _args|
   editor.replace editor.text.upcase
   editor.message "Buffer uppercased by Ruby"