raco setup: add --doc-index; raco pkg: use --doc-index

This combination of changes moves the decision about rebuilding
"scribblings/main" and "scribblings/main/user" to `raco setup',
which is in a better position to know whether documentation should
be built at all.

original commit: 413ca68435

collects/setup/option-sig.rkt

@@ -20,6 +20,7 @@
      make-planet
      avoid-main-installation
      make-tidy
+     make-doc-index
      call-install
      call-post-install
      pause-on-errors