Honestly, you are the first developer that I see that prefers the wiki rather than GitHub for documentation. If you ask around most people feel that the wiki is old fashioned, old technology, so to speak; even cryptic, some people would say. Most people prefer the markdown syntax that GitHub offers, so there has even been talk of migrating the entire documentation to GitHub.
However, since nothing concrete about that has happened, I still think that the wiki is good enough, and not that hard to use.
Thus, my personal recommendation would be to write the documentation in the wiki following the style of the Draft Workbench. I do not like the A2plus Workbench page. It's a single, huge page, with a lot of information. I much prefer the structure that the core workbenches have, which is a central page, and then a page for each of the graphical commands, including instructions to use, example images, and even scripting information.
Take a look at the EM Workbench. It is one of the external workbenches. Its author wrote the documentation in the same style as Draft, PartDesign, TechDraw, etc., so it fits very well with the rest of the documentation.
I would suggest having the documentation only in one place, that is, the wiki, or your GitHub repo. In this way, the chances of having two different versions with diverging content are minimized. I like the wiki because anybody can edit, add information, correct errors, write tutorials, translate, etc. Obviously, you, as the author, should write the most important parts, and then power users who are experienced can add more details. alonso_jamm seems like he would like to work on the Assembly4 pages on the wiki, and I can provide help to make sure it is consistent with the rest of the wiki.
If you decide that the wiki is the "official" documentation, then in your GitHub repo you should provide only a quick example and then guide the reader to the wiki. But if you prefer to keep the documentation on GitHub, that is also valid as well.