scribble/decode: add 'no-index to avoid part titles in index

Using 'no-index makes sense for a book, for example.

scribble-doc/scribblings/scribble/core.scrbl

@@ -451,6 +451,14 @@ The recognized @tech{style properties} are as follows:
        document, causes the HTML output to not include an ``on this 
        page'' margin box.}
 
+ @item{@indexed-racket['no-index] --- Has no effect as a @tech{style
+       property} on a @racket[part], but as a style property on a
+       @racket[title] or @racket[part-start] that provides a
+       @racket[part]'s style via @racket[decode], the
+       @racket['no-index] @tech{style property} cause @racket[decode]
+       to skip the generation of an entry for the part's title in the
+       document index.}
+
  @item{@racket[document-version] structure --- A version number for
        this part and its sub-parts (except as overridden). When it is
        not @racket[""] may be used when rendering a document; at a
@@ -517,7 +525,7 @@ sub-parts).
 
 The @racket[parts] field contains sub-parts.
 
-}
+@history[#:changed "1.25" @elem{Added @racket['no-index] support.}]}
 
 
 @defstruct[paragraph ([style style?] [content content?])]{

scribble-doc/scribblings/scribble/decode.scrbl

@@ -94,8 +94,15 @@ parsing. Instances of @racket[section] trigger are used as-is as
 subsections, and instances of @racket[paragraph] and other
 flow-element datatypes are used as-is in the enclosing flow.
 
+As a part is decoded, as long as the style for the part does not
+include the @tech{style property} @racket['hidden] or
+@racket['no-index], an entry is added to the document index for the
+part's title.
+
 Portions of @racket[lst] are within a part are decoded using
-@racket[decode-flow].}
+@racket[decode-flow].
+
+@history[#:changed "1.25" @elem{Added @racket['no-index] support.}]}
 
 
 @defproc[(decode-part [lst (listof pre-part?)]

scribble-lib/info.rkt

@@ -23,4 +23,4 @@
 
 (define pkg-authors '(mflatt eli))
 
-(define version "1.24")
+(define version "1.25")

scribble-lib/scribble/decode.rkt

@@ -166,8 +166,9 @@
                                                #f))
                          keys k-tags)
                      colls)])
-            (if (and title 
-                     (not (memq 'hidden (style-properties style))))
+            (if (and title
+                     (not (memq 'hidden (style-properties style)))
+                     (not (memq 'no-index (style-properties style))))
               (cons (make-index-element
                      #f null (car tags)
                      (list (clean-up-index-string