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/index.markdown

@@ -0,0 +1,48 @@
+^title Modules
+
+Because Wren can be used both as an embedded scripting language, and as a
+full-featured general purpose programming language run from the command line,
+the definition of a "built-in" module is a little more complicated than other
+languages. They are organized into three categories:
+
+## Core
+
+There is one core module. It is built directly into the VM and is implicitly
+imported by every other module. It contains the classes for the kinds of objects
+built directly into the language itself: [numbers][], [strings][], etc.
+
+[numbers]: core/num.html
+[strings]: core/string.html
+
+The core module is always available and can't be removed.
+
+* [Core](core)
+
+## Optional
+
+Optional modules are available in the command line Wren interpreter. When you
+embed Wren in your own host application, you can also include them too. They are
+written in Wren and C, but have no external dependencies, so including them in
+your application doesn't force you to bring in any other third-party code.
+
+At the same time, they aren't needed by the VM to function, so you can disable
+some or all of them if you want to keep your app as small and constrained as
+possible.
+
+There are a couple of optional modules:
+
+* [Meta](meta)
+* [Random](random)
+
+## CLI
+
+The CLI modules are only available in the standalone command-line Wren
+interpreter. They are deeply tied to [libuv][], each other, and other internals
+of the command-line app, so can't be easily separated out and pulled into host
+applications that want to embed Wren.
+
+[libuv]: http://libuv.org
+
+* [IO](io)
+* [Scheduler](scheduler)
+* [Timer](timer)