real beginner question about D's web site?

[WhatMeWorry via Digitalmars-d-learn]

https://github.com/dlang/dlang.org/blob/ff235feedcb2bcb73ba348dcd1763542a43c7778/doc.ddoc

D_S      = $(LAYOUT ,$1,$(ARGS $+))
SPEC_S   = $(LAYOUT ,$1,$(ARGS $+))
COMMUNITY= $(LAYOUT ,$1,$(ARGS $+))
_=
LAYOUT=$3
_=

I realize that D_S and LAYOUT, etc are macros. I've read the DDoc 
documentation and HTML, CSS, etc. but none of my research 
mentions the $ signs, as well as the $1 and $3. Am I missing 
something obvious?