Start documenting maps and work on some other docs a bit.

doc/site/maps.markdown

@@ -1,5 +1,44 @@
 ^title Maps
 ^category types
 
-Wren will have a built-in hash table type similar to objects in JavaScript and
-tables in Lua, but they haven't been implemented yet.
+A map is an *associative* collection. It holds a set of entries, each of which
+maps a *key* to a *value*. The same data structure has a variety of names in
+other languages: hash table, dictionary, association, table, etc.
+
+You can create a map by placing a series of comma-separated entries inside
+curly braces. Each entry is a key and a value separated by a colon:
+
+    :::dart
+    {
+      "george": "harrison",
+      "john": "lennon",
+      "paul": mccartney",
+      "ringo": "starr"
+    }
+
+This creates a map that maps the first names of the Beatles to their last
+names. Here, we're using strings for both keys and values.
+
+Values can be any Wren object, and multiple keys may map to the same value.
+
+Keys have a few limitations. They must be one of the immutable built-in [value
+types](values.html) in Wren. In other words, a number, string, range, bool, or
+`null`. You can also use a [class object](classes.html) as a key. Any given key
+can only be present in the map once. If you use it twice, the latter replaces
+the former's value.
+
+## Adding entries
+
+**TODO**
+
+## Looking up values
+
+**TODO**
+
+## Removing entries
+
+**TODO**
+
+## Iterating over the contents
+
+**TODO**