chore: reorganize doc site structure by moving core docs under modules/ and removing category metadata

Restructure the documentation hierarchy to prepare for documenting additional built-in modules. Move all core library documentation from `doc/site/core/` to `doc/site/modules/core/`, rename `modules.markdown` to `modularity.markdown`, remove `^category` metadata lines from all affected pages, update cross-reference links to use relative paths under the new structure, and add a TODO placeholder to the Class class page.

doc/site/modules/core/class.markdown

@@ -0,0 +1,29 @@
+^title Class Class
+
+**TODO**
+
+## Methods
+
+### **name**
+
+The name of the class.
+
+### **supertype**
+
+The superclass of this class.
+
+    :::wren
+    class Crustacean {}
+    class Crab is Crustacean {}
+
+    System.print(Crab.supertype) //> Crustacean
+
+A class with no explicit superclass implicitly inherits Object:
+
+    :::wren
+    System.print(Crustacean.supertype) //> Object
+
+Object forms the root of the class hierarchy and has no supertype:
+
+    :::wren
+    System.print(Object.supertype) //> null