Add Perl 6 / Raku resources (#3058)

* Update free-programming-books-es.md

Add Piensa en Perl 6 (Spanish translation of Think Perl 6)

* Update free-programming-books.md

Add another Perl 6 book. Nowadays Perl 6 is also known as Raku (https://marketing.perl6.org/id/1541379592/pdf_digital) and the name Rakudo refers to the only major Perl 6 compiler at the moment. Also update status of 'Using Perl 6'.

* Update free-programming-books.md

Alphabetize list

* add space to satisfy linter

* Update free-programming-books-es.md

free-programming-books-es.md

@@ -34,6 +34,7 @@
 * [NoSQL](#nosql)
   * [MongoDB](#mongodb)
 * [Perl](#perl)
+* [Perl 6 / Raku](#perl-6--raku)
 * [PHP](#php)
 * [Python](#python)
   * [Django](#django)
@@ -287,6 +288,11 @@
 * [Tutoriales de Perl](http://perlenespanol.com/tutoriales/) (HTML)
 
 
+### Perl 6 / Raku
+
+* [Piensa en Perl 6](https://uzluisf.gitlab.io/piensaperl6/) (PDF)
+
+
 ### Python
 
 * [Aprenda a pensar como un programador (con Python)](https://argentinaenpython.com/quiero-aprender-python/aprenda-a-pensar-como-un-programador-con-python.pdf) (PDF)

free-programming-books.md

@@ -155,7 +155,7 @@
 * [OpenResty](#openresty)
 * [OpenSCAD](#openscad)
 * [Perl](#perl)
-* [Perl 6 / Rakudo](#perl-6--rakudo)
+* [Perl 6 / Raku](#perl-6--raku)
 * [PHP](#php)
 * [PicoLisp](#picolisp)
 * [PostgreSQL](#postgresql)
@@ -1948,12 +1948,13 @@ Kerridge (PDF) (email address *requested*, not required)
 * [Web Client Programming with Perl](http://www.oreilly.com/openbook/webclient/)
 
 
-### Perl 6 / Rakudo
+### Perl 6 / Raku
 
+* [Perl 6 at a Glance](https://perl6.online/perl6-at-a-glance/) - Andrew Shitov (HTML) (PDF)
 * [Perl 6 Introduction](http://perl6intro.com)
 * [Perl 6 Programming](https://en.wikibooks.org/wiki/Perl_6_Programming) - Wikibooks
 * [Think Perl 6](http://greenteapress.com/thinkperl6/thinkperl6.pdf) - Laurent Rosenfeld, with Allen B. Downey (PDF)
-* [Using Perl 6](https://github.com/perl6/book/) (:construction: *in process*)
+* [Using Perl 6](https://github.com/perl6/book/) (:construction: *project is dead*)
 
 
 ### PHP