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/null.markdown

@@ -0,0 +1,10 @@
+^title Null Class
+
+## Methods
+
+### **!** operator
+
+Returns `true`, since `null` is considered [false](../control-flow.html#truth).
+
+    :::wren
+    System.print(!null) //> true