Improvements to Introduction chapter

* doc/ref/intro.texi (Introduction): Change "some" to "the".
  (Guile and Scheme): "multithreading" -> "multithreaded".
  (Combining with C): Mention "libguile" as library name.
  (Guile and the GNU Project): New text.
  (Interactive Programming): New text.
  (Supporting Multiple Languages): New text, reflecting currently
  supported languages.
  (Obtaining and Installing Guile): Use @var for version placeholder.
  Remove mention of separate tutorial.