Odoc renders table of content for manuals (.mld files)

[sabine]

Since we eventually want to render the opam documentation using odoc, odoc should render manuals in such a way that the manual pages are discoverable in the UI.

This means that there must be a "table of contents" for the generated manual that allows people to discover the manual pages.

This is related to ocaml-doc/voodoo#47 and has been identified as an issue that is most likely to be resolved here.