Support for package documentation assets

[dbuenzli]

In b0-system/odig@90f8660 I have implemented a first shot at support for documentation assets (used by Vg since this commit dbuenzli/vg@e01ae8e) .

I do however think that odig is the wrong place to standardize this and that the scheme should be in odoc (since without support in the ocamldoc language it influences the links you make in your docs) and be simply driven by odig. As we discussed in b0-system/odig#6, unless something happens quickly at the ocamldoc level I think we should go for something straightforward, usable now.

One idea would be to simply allow to do what odig does at the moment by adding an --assets-dir DIR option to the html command whose semantics is to simply copy over whatever is in DIR in an _assets directory at the root of the package HTML after having generated the HTML. However though that may be a bit build system unfriendly, an odoc asset --pkg PKG file that copies file over to _assets would be more.

Alternatively we could do nothing until there is support in the ocamldoc language/odoc and rely on odig's scheme meanwhile.

[dbuenzli]

This issue becomes more acute now that we have the manuals (#61).