docs/system: document len and index

docs/system.dj

@@ -602,6 +602,29 @@ stroke 128 #00000004 (vec 0 0)
 If you try to to use this brush to fill up a single spot with black, you will notice that despite all the math suggesting so, the color will end up gray instead.
 
 
+## Lists
+
+```haku
+len
+  l : list _
+  -> number
+```
+
+Returns the number of elements in the given list (its `len`gth.)
+
+```haku
+index
+  l : list t
+  i : number
+  -> t
+```
+
+Returns an element of the list, located at the given index `i`.
+The first element is located at index 0.
+
+Indexing out of bounds is an error.
+
+
 ## Shapes
 
 ```haku