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

Change-Id: I9e0a6b5d1a1c820081b3f87c3871068ea042f889
diff --git a/doc/tutorial/basics/disjstruct.md b/doc/tutorial/basics/disjstruct.md
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-[TOC](Readme.md) [Prev](disjunctions.md) [Next](numbers.md)
+[TOC](Readme.md) [Prev](defaults.md) [Next](numbers.md)
+
+_Types and Values_
 
 # Disjunctions of Structs