feat: add category metadata to doc pages and switch code blocks to dart syntax

Add `^category` front matter to all language, types, and reference doc pages for site navigation restructuring. Replace `:::wren` and `:::java` code block annotations with `:::dart` across multiple files. Create new stub pages for core-libraries, embedding-api, and contributing under the reference category. Remove the old embedding and getting-involved stub pages.

doc/site/qa.markdown

@@ -1,4 +1,5 @@
 ^title Q & A
+^category reference
 
 ## Why did you create Wren?
 
@@ -37,7 +38,7 @@ Here's an example of that kind of object-oriented programming in Lua:
 
 Here's the same example in Wren:
 
-    :::wren
+    :::dart
     class Account {
       new(balance) { _balance = balance }
       withdraw(amount) { _balance = _balance - amount }