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/fn.markdown

@@ -0,0 +1,45 @@
+^title Fn Class
+
+A first class function—an object that wraps an executable chunk of code.
+[Here][functions] is a friendly introduction.
+
+[functions]: ../../functions.html
+
+### Fn.**new**(function)
+
+Creates a new function from... `function`. Of course, `function` is already a
+function, so this really just returns the argument. It exists mainly to let you
+create a "bare" function when you don't want to immediately pass it as a [block
+argument](../functions.html#block-arguments) to some other method.
+
+    :::wren
+    var fn = Fn.new {
+      System.print("The body")
+    }
+
+It is a runtime error if `function` is not a function.
+
+## Methods
+
+### **arity**
+
+The number of arguments the function requires.
+
+    :::wren
+    System.print(Fn.new {}.arity)             //> 0
+    System.print(Fn.new {|a, b, c| a }.arity) //> 3
+
+### **call**(args...)
+
+Invokes the function with the given arguments.
+
+    :::wren
+    var fn = Fn.new { |arg|
+      System.print(arg)     //> Hello world
+    }
+
+    fn.call("Hello world")
+
+It is a runtime error if the number of arguments given is less than the arity
+of the function. If more arguments are given than the function's arity they are
+ignored.