Since #585 we no longer install ocamldoc, and this breaks a lot of opam packages (we're only noticing now because #585 broke the opam CI and we didn't fix it until @kit-ty-kate came back last week).

I think we should build it and install it again, as otherwise the opam CI is too noisy to be useful.
If someone familiar with the dune files could look at it (@stedolan, maybe ?), that would be appreciated.