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/system.markdown

@@ -0,0 +1,47 @@
+^title System Class
+
+The System class is a grab-bag of functionality exposed by the VM, mostly for
+use during development or debugging.
+
+## Static Methods
+
+### System.**clock**
+
+Returns the number of seconds (including fractional seconds) since the program
+was started. This is usually used for benchmarking.
+
+### System.**gc**()
+
+Requests that the VM perform an immediate garbage collection to free unused
+memory.
+
+### System.**print**()
+
+Prints a single newline to the console.
+
+### System.**print**(object)
+
+Prints [object] to the console followed by a newline. If not already a string,
+the object is converted to a string by calling `toString` on it.
+
+    :::wren
+    System.print("I like bananas") //> I like bananas
+
+### System.**printAll**(sequence)
+
+Iterates over [sequence] and prints each element, then prints a single newline
+at the end. Each element is converted to a string by calling `toString` on it.
+
+    :::wren
+    System.printAll([1, [2, 3], 4]) //> 1[2, 3]4
+
+### System.**write**(object)
+
+Prints a single value to the console, but does not print a newline character
+afterwards. Converts the value to a string by calling `toString` on it.
+
+    :::wren
+    System.write(4 + 5) //> 9
+
+In the above example, the result of `4 + 5` is printed, and then the prompt is
+printed on the same line because no newline character was printed afterwards.