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

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).