Added initial language reference documentation.
Added lexical structure section.
Added subroutines section to doc/d4-reference.pod
Added if-else section to doc/d4-reference.pod.
Added for-next section to doc/d4-reference.pod.
Added loop-until section to doc/d4-reference.pod.
Added constants section to doc/d4-reference.pod.
Added Input/Output section to doc/d4-reference.pod.