add jekyll-asciidoc (by asciidoctor)

### Before submitting an issue, please be sure to

- [x] Read [the contributing instructions](https://github.com/github/pages-gem/blob/master/docs/CONTRIBUTING.md#contributing-to-the-github-pages-gem)
- [x] Update to the latest Gem version (run `bundle update github-pages`)

### This issue affects

- [x] The site generated by GitHub Pages
- [x] Building sites locally

*The GitHub Pages Gem is intended to help users replicate the GitHub Pages build environment locally. If your issue affects both the hosted version and the version previewed locally, you may be better suited reporting [seeking support in other forums](https://github.com/github/pages-gem/blob/master/docs/SUPPORT.md).*

### What did you do (e.g., steps to reproduce)
Add an `.adoc` file post.

### What did you expect to happen?
Render HTML5.

### What happened instead?
The post didn't render

### Additional information
asciidoctor provides [jekyll-asciidoc](https://github.com/asciidoctor/jekyll-asciidoc). It is quite popular: https://trends.google.com/trends/explore?cat=13&date=today%205-y&geo=CH&q=jekyll,asciidoctor,jemoji and mostly allows for the writing of technical documents, such as standards (see e.g. https://github.com/metanorma/metanorma)

* Link to the live site (if applicable):
* Link to the source repo (if applicable): https://github.com/asciidoctor/jekyll-asciidoc