Add initial documentation for the AsciiDoc reporter

[johthor]

Describe how the AsciiDoc report may be generated via CLI or Maven.

Unfortunately I couldn't describe the Gradle configuration.

The sections for the Maven and Gradle Plugin will be upgraded to level 2 because it didn't make sense to describe them twice. Once for HTML and once for Asciidoc.

Issue: #54

[l-1squared]

⚠️ @johthor can this be merged? I'd like to do a release (we have a bug open that was already fixed on master) and frankly we should have done one already. But I don't want to cut out your contribution from the release (would make a real mess of the commit graph). Rather I think it makes more sense to add this docs with a mark "Experimental"

[l-1squared]

Hi @johthor
I do know that life can easily get in the way of open source contributions. I want to ask, whether you still have this project on the radar. It would be a pity if your significant contribution remained incomplete. It also pains me that I feel I cannot honor your efforts in the Changelog, because that would mean officialy releasing the feature, which I can't because it is incomplete.