Reorganize site to make room for documenting the other built-in modules.

doc/site/modules/core/index.markdown

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+^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