Publish documentation of mullti-opam packages

[gpetiot]

(from #235)

We should only publish the doc of packages that have a doc field pointing to ghpages.

Also since we don't necessarily need that doc uri to be able to publish the doc (ie we can infer a good enough one from the dev-repo) might be worth to allow an escape hatch for people that may want to have their doc field pointing elsewhere but still publish their doc to gh-pages as well.