prefer rakudoc >> pod6

Part of #3785

Author: coke

doc/Language/about.rakudoc

@@ -83,7 +83,7 @@ All of the paragraphs and blocks following that directive, up until the
 next directive of the same level, will be considered part of the
 documentable. So, in:
 
-=begin code :allow<R> :lang<pod6>
+=begin code :allow<R> :lang<rakudoc>
     =head2 R<My Definition>
 
     Some paragraphs, followed by some code:
@@ -152,7 +152,7 @@ with or without code formatting (B<V< C<> >>). Due to current parser
 limitations, special steps have to be taken to use B<V< X<> >> with other
 formatting codes; for example:
 
-=begin code :lang<pod6>
+=begin code :lang<rakudoc>
 =item X<B<foo>|Subroutines,foo> a fancy subroutine
 =end code
 

doc/Language/modules.rakudoc

@@ -802,7 +802,7 @@ structure as the C<lib> directory like so:
 =begin code :lang<text>
 doc
 └── Vortex
-    └── TotalPerspective.pod6
+    └── TotalPerspective.rakudoc
 =end code
 N<Note, described above is a minimal project directory. If your project
 contains scripts that you'd like distributed along with your module(s),

doc/Language/operators.rakudoc

@@ -750,10 +750,10 @@ it independent of the container type.
 =begin code :skip-test<need to mock JSON::Tiny>
 use JSON::Tiny;
 
-my $config = from-json('{ "files": 3, "path": "/home/some-user/raku.pod6" }');
-say $config.raku;      # OUTPUT: «${:files(3), :path("/home/some-user/raku.pod6")}␤»
+my $config = from-json('{ "files": 3, "path": "/home/some-user/raku.rakudoc" }');
+say $config.raku;      # OUTPUT: «${:files(3), :path("/home/some-user/raku.rakudoc")}␤»
 my %config-hash = $config<>;
-say %config-hash.raku; # OUTPUT: «{:files(3), :path("/home/some-user/raku.pod6")}␤»
+say %config-hash.raku; # OUTPUT: «{:files(3), :path("/home/some-user/raku.rakudoc")}␤»
 =end code
 
 It's a C<Hash> in both cases, and it can be used like that; however, in the