Displaying images via raw html is broken for hosted but not local docs

[timholy]

In older versions of Documenter one could display an image like this:

```@raw html
<img src="images/stepin2.png" width="660px"/>
```

This still works when I build the docs locally, but for the hosted docs I get broken links: https://timholy.github.io/Rebugger.jl/dev/usage/.

I'm not sure I'd report this as a bug, except that it worked on older versions of Documenter (see https://timholy.github.io/Rebugger.jl/v0.1.5/usage.html and https://github.com/timholy/Rebugger.jl/pull/53/files).