Ensure tested example code in readme  #219

Will fix problems like #194

1. implement preprocessing asciidoctor files using using https://libraries.io/rubygems/asciidoctor-reducer as mentioned in https://gist.github.com/dcode/0cfbf2699a1fe9b46ff04c41721dda74?permalink_comment_id=3431647#gistcomment-3431647
   to work around missing support for includes in GitHub and possible Yard/Rubydocs
2. extract source code snippets from README to separate test files and include them back via asciidoctor
3. ensure github links like gh-229 Port CI from travis-CI to GitHub actions #229 work
4. later (separate task?) update the keyword usage in the examples to Ruby 3.0 syntax