Upgrade to latest AsciiDoctor Editor

Multiple new versions were released, see https://github.com/de-jcup/eclipse-asciidoctor-editor/releases. Currently the latest one is 2.4.1, see https://github.com/de-jcup/eclipse-asciidoctor-editor/releases/tag/v2.4.1.

We should upgrade to the latest version.

Then we should check whether there are warnings/errors when we open our AsciiDoc files, whether images are correctly show, etc. We should report any issues we find, such that they can be addressed.