Open rdoc/index.html in your browser and start exploring.
I want to look into the server-side rendering of graphgists and instead doing the jquery selection on the rendered HTML I thought it would much easier and faster to access those elements and information during the asciidoctor rendering process.
Absolutely. I think that postprocessing HTML should always be considered the last resort. That usually means that an extension point wasn't available or wasn't sufficient to meet the requirements. Of course, there are times when postprocessing the HTML is appropriate, but it should be the exception rather than the rule.
So if you have any pointers about the API, visitors, callbacks, datastructures or examples that I could look into, I would very much appreciate that.
The Extension API docs are the most complete in all of Asciidoctor, so I certainly recommend reading through those. The main data model is the AbstractNode and AbstractBlock. You'll notice that Asciidoctor mostly uses a "schemaless" model to keep things simple and flexible...so you won't find specific types for each block context (i.e., sidebar, listing, etc). Most of the data for a block gets stored in its attributes.
You can also check out the Asciidoctor extension labs to get ideas about where to tap into the parsing:
Re: access to the document components during or after rendering
I almost forgot to mention. We plan to make all the extension points available in AsciidoctorJ and Asciidoctor.js as well. In fact, they are already available, but we continue to work to improve the accessibility in those environments for extension writers.