Merge remote-tracking branch 'origin/CGAL-url_of_previous_manuals-GF'

- Remove the paragraph "This is the beta version of the CGAL Manual
  generated with doxygen." that was in the main page of the CGAL manual.

- Add a section, at the end of the main page, that explains where to
  find the manuals of previous CGAL versions.

Documentation/doc/Documentation/main.txt

@@ -2,16 +2,7 @@
 
 \mainpage
 
-This is the beta version of the %CGAL Manual generated with doxygen.
- As it was generated semi-automatically from LaTeX sources of an
-inhouse documentation tool, followed by months of postprocessing
-we are happy about user feedback how we might improve things and
-where we have documentation bugs.
 
-For that we have set up a [tracker](https://gforge.inria.fr/tracker/?atid=13845&group_id=52&func=browse) where you can submit documentation bugs.
-
-During beta we only provide the manual online, so that we can
-publish fixes quickly.
 
 About %CGAL
 ==========
@@ -78,6 +69,16 @@ other contexts can be done by obtaining a commercial license from
 [GeometryFactory](http://www.geometryfactory.com). For more details
 see the \ref licenseIssues "License" page.
 
+Manuals for the Previous Releases
+=================================
+
+For releases >= 4.2, visit [http://doc.cgal.org/X.Y](http://doc.cgal.org/4.2)
+
+For releases X.Y, with 3.1 <= X.Y <= 4.1 visit
+[http://www.cgal.org/Manual/X.Y/doc_html/cgal_manual/contents.html](http://www.cgal.org/Manual/3.1/doc_html/cgal_manual/contents.html)
+
+
+
 \htmlonly
 <div style="display:none">
 \endhtmlonly