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/fiber.markdown

@@ -0,0 +1,141 @@
+^title Fiber Class
+
+A lightweight coroutine. [Here][fibers] is a gentle introduction.
+
+[fibers]: ../../concurrency.html
+
+### Fiber.**new**(function)
+
+Creates a new fiber that executes `function` in a separate coroutine when the
+fiber is run. Does not immediately start running the fiber.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print("I won't get printed")
+    }
+
+## Static Methods
+
+### Fiber.**current**
+
+The currently executing fiber.
+
+### Fiber.**suspend**()
+
+Pauses the current fiber, and stops the interpreter. Control returns to the
+host application.
+
+To resume execution, the host application will need to invoke the interpreter
+again. If there is still a reference to the suspended fiber, it can be resumed.
+
+### Fiber.**yield**()
+
+Pauses the current fiber and transfers control to the parent fiber. "Parent"
+here means the last fiber that was started using `call` and not `run`.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print("Before yield")
+      Fiber.yield()
+      System.print("After yield")
+    }
+
+    fiber.call()                //> Before yield
+    System.print("After call")  //> After call
+    fiber.call()                //> After yield
+
+When resumed, the parent fiber's `call()` method returns `null`.
+
+If a yielded fiber is resumed by calling `call()` or `run()` with an argument,
+`yield()` returns that value.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print(Fiber.yield()) //> value
+    }
+
+    fiber.call()        // Run until the first yield.
+    fiber.call("value") // Resume the fiber.
+
+If it was resumed by calling `call()` or `run()` with no argument, it returns
+`null`.
+
+If there is no parent fiber to return to, this exits the interpreter. This can
+be useful to pause execution until the host application wants to resume it
+later.
+
+    :::wren
+    Fiber.yield()
+    System.print("this does not get reached")
+
+### Fiber.**yield**(value)
+
+Similar to `Fiber.yield` but provides a value to return to the parent fiber's
+`call`.
+
+    :::wren
+    var fiber = Fiber.new {
+      Fiber.yield("value")
+    }
+
+    System.print(fiber.call()) //> value
+
+## Methods
+
+### **call**()
+
+Starts or resumes the fiber if it is in a paused state.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print("Fiber called")
+      Fiber.yield()
+      System.print("Fiber called again")
+    }
+
+    fiber.call() // Start it.
+    fiber.call() // Resume after the yield() call.
+
+When the called fiber yields, control is transferred back to the fiber that
+called it.
+
+If the called fiber is resuming from a yield, the `yield()` method returns
+`null` in the called fiber.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print(Fiber.yield())
+    }
+
+    fiber.call()
+    fiber.call() //> null
+
+### **call**(value)
+
+Invokes the fiber or resumes the fiber if it is in a paused state and sets
+`value` as the returned value of the fiber's call to `yield`.
+
+    :::wren
+    var fiber = Fiber.new {
+      System.print(Fiber.yield())
+    }
+
+    fiber.call()
+    fiber.call("value") //> value
+
+### **isDone**
+
+Whether the fiber's main function has completed and the fiber can no longer be
+run. This returns `false` if the fiber is currently running or has yielded.
+
+### **transfer**()
+
+**TODO**
+
+### **transfer**(value)
+
+**TODO**
+
+### **transferError**(error)
+
+**TODO**