Add references to class-level documentation

Author: Owen Jones

doc/architectural/data-structures-core-structures-and-ast.md

@@ -36,10 +36,14 @@ considered important, for example it shouldn't be counted when comparing two
 
 ## typet ##
 
+See \ref typet.
+
 To be documented.
 
 ### symbol_typet ###
 
+See \ref symbol_typet.
+
 To be documented.
 
 ## exprt ##