Rakudoc - make the naming consistent

[Richard Hainsworth]

With the change from Perl 6 to Raku, there were a number of naming 
changes, particularly of file formats, eg pl6 -> raku, pm6 -> rakumod.

One of the changes was `.pod6` to `.rakudoc` for files that are 
primarily documentation files.

However, we continue to refer to POD6 (also known as Pod6) as the name 
of the markup language for Raku documentation. POD6 comes from Perl's 
markup language and comes from Plain Old Documentation, together with 
the 6 version.

My recommendation is to rename the markup language as `Rakudoc`. This is 
consistent with the file format name.

This is a minor change and will not affect many people at all. It will 
not affect the operation of any program constructs.

However, without a consensus, references to the markup language as 
Rakudoc instead of POD6 might cause confusion.