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/index.markdown

@@ -0,0 +1,24 @@
+^title Core Module
+
+Because Wren is designed for [embedding in applications][embedding], its core
+module is minimal and is focused on working with objects within Wren. For
+stuff like file IO, graphics, etc., it is up to the host application to provide
+interfaces for this.
+
+All Wren source files automatically have access to the following classes:
+
+* [Bool](bool.html)
+* [Class](class.html)
+* [Fiber](fiber.html)
+* [Fn](fn.html)
+* [List](list.html)
+* [Map](map.html)
+* [Null](null.html)
+* [Num](num.html)
+* [Object](object.html)
+* [Range](range.html)
+* [Sequence](sequence.html)
+* [String](string.html)
+* [System](system.html)
+
+[embedding]: ../../embedding-api.html