Docs: list all libraries in top-level index
Docs: list all libraries in top-level index

This commit changes the top-level index of the docs generated by
Jbuilder so that the index lists all installed libraries (Findlib
library packages generated by Jbuilder), rather than only opam packages.
For example, current Markup.ml installs libraries

  markup
  markup.lwt
  markup.lwt.unix

With this change, all these libraries are listed in the index. Before
this change, only markup was listed, because it happened to coincide
with the name of the opam package that contains all three libraries.

Not listing the extra libraries makes them undiscoverable, as they can
only be reached by direct URL.

janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,rdavison/jbuilder,dra27/jbuilder,janestreet/jbuilder,rdavison/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder