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/range.markdown

@@ -0,0 +1,57 @@
+^title Range Class
+
+A range defines a bounded range of values from a starting point to a possibly
+exclusive endpoint. [Here](../range.html) is a friendly introduction.
+
+Extends [Sequence](sequence.html).
+
+## Methods
+
+### **from**
+
+The starting point of the range. A range may be backwards, so this can be
+greater than [to].
+
+    :::wren
+    System.print((3..5).min) //> 3
+    System.print((4..2).min) //> 4
+
+### **to**
+
+The endpoint of the range. If the range is inclusive, this value is included,
+otherwise it is not.
+
+    :::wren
+    System.print((3..5).min) //> 5
+    System.print((4..2).min) //> 2
+
+### **min**
+
+The minimum bound of the range. Returns either `from`, or `to`, whichever is
+lower.
+
+    :::wren
+    System.print((3..5).min) //> 3
+    System.print((4..2).min) //> 2
+
+### **max**
+
+The maximum bound of the range. Returns either `from`, or `to`, whichever is
+greater.
+
+    :::wren
+    System.print((3..5).min) //> 5
+    System.print((4..2).min) //> 4
+
+### **isInclusive**
+
+Whether or not the range includes `to`. (`from` is always included.)
+
+    :::wren
+    System.print((3..5).isInclusive)   //> true
+    System.print((3...5).isInclusive)  //> false
+
+### **iterate**(iterator), **iteratorValue**(iterator)
+
+Iterates over the range. Starts at `from` and increments by one towards `to`
+until the endpoint is reached.