Adds a little introduction to the 'language' page

Which is the most visited in the site. Not the best, but brief and to
the point. Closes #1884

Author: JJ

htmlify.p6

@@ -833,6 +833,7 @@ sub write-index-files() {
     say 'Writing html/language.html ...';
     spurt 'html/language.html', p2h(pod-with-title(
         'Perl 6 Language Documentation',
+        pod-block("Tutorials, general reference, migration guides and meta pages for the Perl 6 language, in alphabetical order. Scroll down or search 'tutorial' or 'from' to see all of them."),
         pod-table($*DR.lookup('language', :by<kind>).sort(*.name).map({[
             pod-link(.name, .url),
             .summary