Streamline the manual's Preface material

Streamline the manual's Preface material

(The principle here is that Preface should only contain
meta-information about the manual, such as how to use it and who wrote
it; plus any political points that we want to draw attention to.)

* doc/ref/preface.texi (Preface): Make unnumbered, for consistency
  with other GNU manuals (e.g. Emacs, GSL).  Merge layout text into
  top level.  Simplify first sentence.  Move section on conventions
  away...

* doc/ref/intro.texi (Introduction to Guile): ...to here.