docs: add core library overview, map class docs, and link value types to core classes

doc/site/core/index.markdown

@@ -1,4 +1,24 @@
 ^title Core Library
 ^category core
 
-**TODO**
+Because Wren is designed for [embedding in applications][embedding], its core
+library 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)
+
+[embedding]: ../embedding-api.html