Doccheck to check local links

[mortenpi]

Right now, we can check remote links with linkcheck, but not local links. We will warn in HTMLWriter if a local link is broken (e.g. an image link pointing a non-existent file in src/), but you can't turn those into errors.

I think we should just add a new doccheck for this that performs the check a little bit earlier (at the same time as linkcheck I guess). This would allow users to catch errors where you have e.g. local images links in your manual etc. that can not be resolved.

Would be good to get this into 1.0, so that we could enable this by default in strict mode.