change Scribble HTML renderer to typeset em dashes with &8212;

Closes PR 11049
 Closes PR 11539

collects/scribble/html-render.rkt

@@ -1263,7 +1263,7 @@
              (ascii-ize i)))]
         [(symbol? i)
          (case i
-           [(mdash) '(" " ndash " ")]
+           [(mdash) '(8212 (wbr))] ;; <wbr> encourages breaking after rather than before
            ;; use "single left/right-pointing angle quotation mark"
            ;; -- it's not a correct choice, but works best for now
            ;;    (see the "Fonts with proper angle brackets"

collects/scribblings/scribble/decode.scrbl

@@ -19,9 +19,7 @@ special text conversions:
 
 @itemize[
 
- @item{@litchar{---}: converted to @racket['mdash], which the HTML render
-       outputs as an en-dash surrounded by space (so don't put spaces around
-       @litchar{---} in a document)}
+ @item{@litchar{---}: converted to @racket['mdash]}
 
  @item{@litchar{--}: converted to @racket['ndash]}