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

doc/site/core/list.markdown

@@ -7,7 +7,7 @@ An indexable contiguous collection of elements. More details [here](../lists.htm
 
 ### **add**(item)
 
-Appends `item` onto the end of the list.
+Appends `item` to the end of the list.
 
 ### **clear**
 
@@ -28,12 +28,36 @@ for iterating over the elements in the list.
 
 ### **removeAt**(index)
 
-**TODO**
+Removes the element at `index`. If `index` is negative, it counts backwards
+from the end of the list where `-1` is the last element. All trailing elements
+are shifted up to fill in where the removed element was.
+
+    :::dart
+    var list = ["a", "b", "c", "d"]
+    list.removeAt(1)
+    IO.print(list) // "[a, c, d]".
+
+It is a runtime error if the index is not an integer or is out of bounds.
 
 ### **[**index**]** operator
 
-**TODO**
+Gets the element at `index`. If `index` is negative, it counts backwards from
+the end of the list where `-1` is the last element.
+
+    :::dart
+    var list = ["a", "b", "c"]
+    IO.print(list[1]) // "b".
+
+It is a runtime error if the index is not an integer or is out of bounds.
 
 ### **[**index**]=**(item) operator
 
-**TODO**
+Replaces the element at `index` with `item`. If `index` is negative, it counts
+backwards from the end of the list where `-1` is the last element.
+
+    :::dart
+    var list = ["a", "b", "c"]
+    list[1] = "new"
+    IO.print(list) // "[a, new, c]".
+
+It is a runtime error if the index is not an integer or is out of bounds.

doc/site/core/map.markdown

@@ -0,0 +1,53 @@
+^title Map Class
+^category core
+
+An associative collection that maps keys to values. More details [here](../maps.html).
+
+### **clear**
+
+Removes all entries
+
+### **count**
+
+The number of entries in the map.
+
+### **keys**
+
+A [Sequence](sequence.html) that can be used to iterate over the keys in the
+map. Note that iteration order is undefined. All keys will be iterated over,
+but may be in any order, and may even change between invocations of Wren.
+
+### **iterate**(iterator), **iteratorValue**(iterator)
+
+Implements the [iterator protocol](../control-flow.html#the-iterator-protocol)
+for iterating over the elements in the list.
+
+### **removeAt**(key)
+
+### **values**
+
+A [Sequence](sequence.html) that can be used to iterate over the values in the
+map. Note that iteration order is undefined. All values will be iterated over,
+but may be in any order, and may even change between invocations of Wren.
+
+If multiple keys are associated with the same value, the value will appear
+multiple times in the sequence.
+
+### **[**key**]** operator
+
+Gets the value associated with `key` in the map. If `key` is not present in the
+map, returns `null`.
+
+    :::dart
+    var map = {"george": "harrison", "ringo": "starr"}
+    IO.print(map["ringo"]) // "starr".
+    IO.print(map["pete"])  // "null".
+
+### **[**key**]=**(value) operator
+
+Associates `value` with `key` in the map. If `key` was already in the map, this
+replaces the previous association.
+
+It is a runtime error if the key is not a [Bool](bool.html),
+[Class](class.html), [Null](null.html), [Num](num.html), [Range](range.html),
+or [String](string.html).