Move Scheme introduction (Guile-independent) to its own chapter

This magically fixes all the internal wordings that say "chapter"
rather than "section", and separates Guile-independent and skippable
material from material that is Guile-specific and
probably-not-skippable - which I think is helpful.

* doc/ref/guile.texi (Top): New `Hello Scheme!' menu item, and removed
  `Basic Ideas' item.  Move include of scheme-ideas.texi before
  `Programming in Scheme' chapter, using @raisesections and
  @lowersections.  Also move scheme-reading.texi to same place.

* doc/ref/scheme-ideas.texi: Change name from `Basic Ideas' (which is
  a bit boring) to `Hello Scheme!'.  Move `Further Reading' menu item
  here.

doc/ref/guile.texi

@@ -169,6 +169,8 @@ x
 * Preface::
 * Introduction::
 
+* Hello Scheme!::
+
 * Programming in Scheme::
 * Programming in C::
 
@@ -203,6 +205,11 @@ Indices
 
 @include intro.texi
 
+@raisesections
+@include scheme-ideas.texi
+@lowersections
+@include scheme-reading.texi
+
 @node Programming in Scheme
 @chapter Programming in Scheme
 
@@ -222,19 +229,15 @@ etc. that make up Guile's application programming interface (API),
 @xref{API Reference}.
 
 @menu
-* Basic Ideas::                 Basic ideas in Scheme.
 * Guile Scheme::                Guile's implementation of Scheme.
 * Guile Scripting::             How to write Guile scripts.
 * Using Guile Interactively::   Guile's REPL features.
 * Using Guile in Emacs::        Guile and Emacs.
-* Further Reading::             Where to find out more about Scheme.
 @end menu
 
-@include scheme-ideas.texi
 @include scheme-intro.texi
 @include scheme-scripts.texi
 @include scheme-using.texi
-@include scheme-reading.texi
 
 @node Programming in C
 @chapter Programming in C

doc/ref/scheme-ideas.texi

@@ -4,8 +4,8 @@
 @c   Free Software Foundation, Inc.
 @c See the file guile.texi for copying conditions.
 
-@node Basic Ideas
+@node Hello Scheme!
-@section Basic Ideas in Scheme
+@chapter Hello Scheme!
 
 In this chapter, we introduce the basic concepts that underpin the
 elegance and power of the Scheme language.
@@ -25,6 +25,7 @@ tutorial.
 * About Procedures::            The representation and use of procedures.
 * About Expressions::           All kinds of expressions and their meaning.
 * About Closure::               Closure, scoping and environments.
+* Further Reading::             Where to find out more about Scheme.
 @end menu