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/bool.markdown

@@ -0,0 +1,19 @@
+^title Bool Class
+
+Boolean [values][]. There are two instances, `true` and `false`.
+
+[values]: ../../values.html
+
+## Methods
+
+### **!** operator
+
+Returns the logical complement of the value.
+
+    :::wren
+    System.print(!true) //> false
+    System.print(!false) //> true
+
+### toString
+
+The string representation of the value, either `"true"` or `"false"`.