(Version 0.22.X) Support for listing blocks and additional diagram types

(Version 0.22.X) Support for listing blocks and additional diagram types

New features

Added example code of a folder structure listing to the AsciiDoc Showroom.
Fixed an error in the description of a code block in the AsciiDoc Showroom.
Added example code of a Meme "diagram" to the AsciiDoc Showroom.
Added some general information about the files and the project to the README file.
Added notes about the usage of passthrough blocks to the AsciiDoc Showroom.
Version 0.22.1: Corrected and extended the release notes for version 0.12.0 regarding the RSS/Atom feed feature. Also fixed the MIME-type in the HTML layout file from application/rss+xml to application/atom+xml.
Version 0.22.2: Added a code example to the AsciiDoc Showroom demonstrating how to use theme assets from jekyll-theme-endless within the Asciidoctor diagram extension meme during the build process.
- For this, enable Liquid code processing within AsciiDoc documents using :page-liquid:.
- After that, the location of the source image in the build directory can be inserted into the AsciiDoc document via the Jekyll variable destination, using the Liquid variable syntax {{ site.destination }}.
Version 0.22.3: The example for using files from the dependencies was removed to avoid increasing the dependency on Jekyll. It should always be possible to build the documents independently of Jekyll, for example with asciidoctor-pdf.