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/object.markdown

@@ -0,0 +1,53 @@
+^title Object Class
+
+## Static Methods
+
+### **same**(obj1, obj2)
+
+Returns `true` if *obj1* and *obj2* are the same. For [value
+types](../values.html), this returns `true` if the objects have equivalent
+state. In other words, numbers, strings, booleans, and ranges compare by value.
+
+For all other objects, this returns `true` only if *obj1* and *obj2* refer to
+the exact same object in memory.
+
+This is similar to the built in `==` operator in Object except that this cannot
+be overriden. It allows you to reliably access the built-in equality semantics
+even on user-defined classes.
+
+## Methods
+
+### **!** operator
+
+Returns `false`, since most objects are considered [true][].
+
+[true]: control-flow.html#truth
+
+### **==**(other) and **!=**(other) operators
+
+Compares two objects using built-in equality. This compares [value
+types](../values.html) by value, and all other objects are compared by
+identity—two objects are equal only if they are the exact same object.
+
+### **is**(class) operator
+
+Returns `true` if this object's class or one of its superclasses is `class`.
+
+    :::wren
+    System.print(123 is Num)     //> true
+    System.print("s" is Num)     //> false
+    System.print(null is String) //> false
+    System.print([] is List)     //> true
+    System.print([] is Sequence) //> true
+
+It is a runtime error if `class` is not a [Class][].
+
+### **toString**
+
+A default string representation of the object.
+
+### **type**
+
+The [Class][] of the object.
+
+[class]: class.html