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

Change-Id: I9e0a6b5d1a1c820081b3f87c3871068ea042f889
diff --git a/doc/tutorial/basics/disjunctions.md b/doc/tutorial/basics/disjunctions.md
index 11fd875..6831dba 100644
--- a/doc/tutorial/basics/disjunctions.md
+++ b/doc/tutorial/basics/disjunctions.md
@@ -1,5 +1,7 @@
 [TOC](Readme.md) [Prev](unification.md) [Next](disjstruct.md)
 
+_Types and Values_
+
 # Disjunctions
 
 Disjunctions, or sum types, define a new type that is one of several things.