Docs: use imagedir asciidoc

We should be setting the location of docs with imagedir as per: https://asciidoctor.org/docs/user-manual/#setting-the-location-of-images

I didn't know you could set ":imagesdir: path" in the main file and it applies to every image target. This makes things cleaner.

Need to add "imagesdir" to each .asciidoc for generating individual titles and /documentation/pom.xml for building all titles with maven.

Also fix a broken image in the cache modes comparison topic. Break after "+"