support "next" value for `page-side-start`

quad/quad/scribblings/quad.scrbl

@@ -546,7 +546,7 @@ First page number used. Value is an integer. Default is @racket[1].
 }
 
 @defthing[#:kind "attribute" page-side-start symbol?]{
-Side that first page appears on. Value is @racket["left"] or @racket["right"]. A blank page will be inserted if necessary. Default is @racket["right"].
+Side that first page appears on. Value is @racket["left"], @racket["right"], or @racket["next"]. Default is @racket["right"]. If the value is @racket["left"] or @racket["right"], a blank page will be inserted if necessary.
 }
 
 @deftogether[(@defthing[#:kind "attribute" column-count symbol?]

quad/quadwriter/render.rkt

@@ -351,8 +351,11 @@
     (define insert-blank-page?
       (and (pair? qs)
            ;; if we need a 'left page and will get 'right (or vice versa) then insert page
-           (let ([next-page-side (if (even? (add1 (section-pages-used))) 'left 'right)])
-             (not (eq? section-starting-side next-page-side)))))
+           (cond
+             [(eq? section-starting-side 'next) #false]
+             [else
+              (let ([next-page-side (if (even? (add1 (section-pages-used))) 'left 'right)])
+                (not (eq? section-starting-side next-page-side)))])))
 
     ;; update page count before starting page wrap
     (when insert-blank-page?