Scribble manual: make "Images" section in ToC be useful

Remove the "Images" section for `scribble/manual`, add an "Images"
section for `scribble/base`.

As suggested by Shriram.

original commit: 92e00de9f3a8257a5434ec15030d60241940483e

pkgs/scribble-pkgs/scribble-doc/scribblings/scribble/base.scrbl

@@ -467,6 +467,9 @@ which renders as
 
 See also @racket[verbatim].}
 
+@; ------------------------------------------------------------------------
+@section[#:tag "images"]{Images}
+
 
 @defproc[(image [path (or/c path-string? (cons/c 'collects (listof bytes?)))]
                 [#:scale scale real? 1.0]

pkgs/scribble-pkgs/scribble-doc/scribblings/scribble/manual.scrbl

@@ -1781,7 +1781,7 @@ the link.}
 @; ------------------------------------------------------------------------
 @section[#:tag "manual-indexing"]{Indexing}
 
-See also @secref["base-indexing"].
+See also @secref["base-indexing"] for @racketmodname[scribble/base].
 
 @defform[(indexed-racket datum ...)]{
 
@@ -1819,15 +1819,6 @@ key for the index iterm does not include quotes.}
 @defproc[(indexed-envvar [pre-content pre-content?] ...) element?]{A
 combination of @racket[envvar] and @racket[as-index].}
 
-@; ------------------------------------------------------------------------
-@section[#:tag "manual-images"]{Images}
-
-@defproc[(image/plain [filename-relative-to-source string?]
-                      [pre-element any/c] ...)
-         element?]{
-
- An alias for @racket[image] for backward compatibility.}
-
 @; ------------------------------------------------------------------------
 @section{Bibliography}
 
@@ -1992,6 +1983,11 @@ described by @racket[what]), where @racket[replacement] describes a
 suitable replacement. The @racket[additional-notes] are included after the
 initial deprecation message.}
 
+@defproc[(image/plain [filename-relative-to-source string?]
+                      [pre-element any/c] ...)
+         element?]{
+
+ An alias for @racket[image] for backward compatibility.}
 
 @; ------------------------------------------------------------------------
 @section[#:tag "index-entries"]{Index-Entry Descriptions}