doc/tutorial/basics: add per-page chapter headings

Change-Id: I9e0a6b5d1a1c820081b3f87c3871068ea042f889
diff --git a/doc/tutorial/basics/imports.md b/doc/tutorial/basics/imports.md
index c6fdf59..025b47b 100644
--- a/doc/tutorial/basics/imports.md
+++ b/doc/tutorial/basics/imports.md
@@ -1,5 +1,7 @@
 [TOC](Readme.md) [Prev](packages.md) [Next](operators.md)
 
+_Modules, Packages, and Instances_
+
 # Imports
 
 A CUE file may import definitions from builtin or user-defined packages.