Removes old single page perl6.xhtml, adds redirect, closes #1956

Author: JJ

html/.htaccess

@@ -30,6 +30,7 @@ RewriteRule ^type/(.*?)\$REVERSE_SOLIDUS(.*) /type/$1%5c$2 [N]
 
 Redirect 301 /language/5to6 https://docs.perl6.org/language/5to6-nutshell
 Redirect 301 /examples      http://examples.perl6.org/
+Redirect 301 /perl6.xhtml   https://docs.perl6.org/perl6.html
 Redirect 301 /examples.html http://examples.perl6.org/
 Redirect 303 /webchat       https://webchat.freenode.net/?channels=#perl6
 Redirect 303 /webchat.html  https://webchat.freenode.net/?channels=#perl6