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/num.markdown

@@ -0,0 +1,163 @@
+^title Num Class
+
+## Static Methods
+
+### Num.**fromString**(value)
+
+Attempts to parse `value` as a decimal literal and return it as an instance of
+`Num`. If the number cannot be parsed `null` will be returned.
+
+It is a runtime error if `value` is not a string.
+
+### Num.**pi**
+
+The value of π.
+
+## Methods
+
+### **abs**
+
+The absolute value of the number.
+
+    :::wren
+    System.print(-123.abs) //> 123
+
+### **acos**
+
+The arc cosine of the number.
+
+### **asin**
+
+The arc sine of the number.
+
+### **atan**
+
+The arc tangent of the number.
+
+### **atan**(x)
+
+The arc tangent of the number when divided by `x`, using the signs of the two
+numbers to determine the quadrant of the result.
+
+### **ceil**
+
+Rounds the number up to the nearest integer.
+
+    :::wren
+    System.print(1.5.ceil)    //> 2
+    System.print((-3.2).ceil) //> -3
+
+### **cos**
+
+The cosine of the number.
+
+### **floor**
+
+Rounds the number down to the nearest integer.
+
+    :::wren
+    System.print(1.5.floor)    //> 1
+    System.print((-3.2).floor) //> -4
+
+### **isInfinity**
+
+Whether the number is positive or negative infinity or not.
+
+    :::wren
+    System.print(99999.isInfinity)  //> false
+    System.print((1/0).isInfinity)  //> true
+
+### **isInteger**
+
+Whether the number is an integer or has some fractional component.
+
+    :::wren
+    System.print(2.isInteger)   //> true
+    System.print(2.3.isInteger) //> false
+
+### **isNan**
+
+Whether the number is [not a number](http://en.wikipedia.org/wiki/NaN). This is
+`false` for normal number values and infinities, and `true` for the result of
+`0/0`, the square root of a negative number, etc.
+
+### **sin**
+
+The sine of the number.
+
+### **sqrt**
+
+The square root of the number. Returns `nan` if the number is negative.
+
+### **tan**
+
+The tangent of the number.
+
+### **-** operator
+
+Negates the number.
+
+    :::wren
+    var a = 123
+    System.print(-a) //> -123
+
+### **-**(other), **+**(other), **/**(other), **\***(other) operators
+
+The usual arithmetic operators you know and love. All of them do 64-bit
+floating point arithmetic. It is a runtime error if the right-hand operand is
+not a number. Wren doesn't roll with implicit conversions.
+
+### **%**(denominator) operator
+
+The floating-point remainder of this number divided by `denominator`.
+
+It is a runtime error if `denominator` is not a number.
+
+### **<**(other), **>**(other), **<=**(other), **>=**(other) operators
+
+Compares this and `other`, returning `true` or `false` based on how the numbers
+are ordered. It is a runtime error if `other` is not a number.
+
+### **~** operator
+
+Performs *bitwise* negation on the number. The number is first converted to a
+32-bit unsigned value, which will truncate any floating point value. The bits
+of the result of that are then negated, yielding the result.
+
+### **&**(other) operator
+
+Performs bitwise and on the number. Both numbers are first converted to 32-bit
+unsigned values. The result is then a 32-bit unsigned number where each bit is
+`true` only where the corresponding bits of both inputs were `true`.
+
+It is a runtime error if `other` is not a number.
+
+### **|**(other) operator
+
+Performs bitwise or on the number. Both numbers are first converted to 32-bit
+unsigned values. The result is then a 32-bit unsigned number where each bit is
+`true` only where the corresponding bits of both inputs were `true`.
+
+It is a runtime error if `other` is not a number.
+
+### **..**(other) operator
+
+Creates a [Range](range.html) representing a consecutive range of numbers
+from the beginning number to the ending number.
+
+    :::wren
+    var range = 1.2..3.4
+    System.print(range.min)         //> 1.2
+    System.print(range.max)         //> 3.4
+    System.print(range.isInclusive) //> true
+
+### **...**(other) operator
+
+Creates a [Range](range.html) representing a consecutive range of numbers
+from the beginning number to the ending number not including the ending number.
+
+    :::wren
+    var range = 1.2...3.4
+    System.print(range.min)         //> 1.2
+    System.print(range.max)         //> 3.4
+    System.print(range.isInclusive) //> false