docs: revise branching, classes, index, looping, qa, syntax docs and add concurrency/error-handling nav entries

doc/site/classes.markdown

@@ -9,7 +9,7 @@ Classes contain both *behavior* and *state*. Behavior is defined in *methods* wh
 
 Classes are created using the `class` keyword, unsurprisingly:
 
-    :::wren
+    :::dart
     class Unicorn {}
 
 This creates a class named `Unicorn` with no methods or fields.
@@ -18,35 +18,35 @@ This creates a class named `Unicorn` with no methods or fields.
 
 To make our unicorn do stuff, we need to give it methods.
 
-    :::wren
+    :::dart
     class Unicorn {
       prance {
-        IO.write("The unicorn prances in a fancy manner!")
+        IO.print("The unicorn prances in a fancy manner!")
       }
     }
 
 This defines a `prance` method that takes no arguments. To support parameters, add a parenthesized parameter list after the method's name:
 
-    :::wren
+    :::dart
     // Inside class...
     prance(where, when) {
-      IO.write("The unicorn prances in " + where + " at " + when)
+      IO.print("The unicorn prances in " + where + " at " + when)
     }
 
 Unlike most other dynamically-typed languages, in Wren you can have multiple methods in a class with the same name, as long as they take a different number of parameters. In other words, you can overload by arity. So this class is fine:
 
-    :::wren
+    :::dart
     class Unicorn {
       prance {
-        IO.write("The unicorn prances in a fancy manner!")
+        IO.print("The unicorn prances in a fancy manner!")
       }
 
       prance(where) {
-        IO.write("The unicorn prances in " + where)
+        IO.print("The unicorn prances in " + where)
       }
 
       prance(where, when) {
-        IO.write("The unicorn prances in " + where + " at " + when)
+        IO.print("The unicorn prances in " + where + " at " + when)
       }
     }
 
@@ -72,7 +72,7 @@ A class can inherit from a "parent" or *superclass*. When you invoke a method on
 
 By default, any new class inherits from `Object`, which is the superclass from which all other classes ultimately descend. You can specify a different parent class using `is` when you declare the class:
 
-    :::class
+    :::dart
     class Pegasus is Unicorn {}
 
 This declares a new class `Pegasus` that inherits from `Unicorn`.