Add pointers to the Guide in Reference docs.

collects/scribblings/reference/data.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:style 'toc #:tag "data"]{Datatypes}
 
+@guideintro["datatypes"]{Datatypes}
+
 Each pre-defined datatype comes with a set of procedures for
 manipulating instances of the datatype.
 

collects/scribblings/reference/io.scrbl

@@ -3,6 +3,8 @@
 
 @title[#:tag "input-and-output" #:style 'toc]{Input and Output}
 
+@guideintro["i/o"]{Ports and I/O}
+
 @local-table-of-contents[]
 
 @include-section["ports.scrbl"]

collects/scribblings/reference/macros.scrbl

@@ -3,12 +3,15 @@
 
 @title[#:style 'toc]{Macros}
 
+@guideintro["macros"]{Macros}
+
 See @secref["syntax-model"] for general information on how programs
 are parsed. In particular, the subsection @secref["expand-steps"]
 describes how parsing triggers macros, and
 @secref["transformer-model"] describes how macro transformers are
 called.
 
+
 @local-table-of-contents[]
 
 @include-section["stx-patterns.scrbl"]