Build setup with docker images

❗ This major update includes docker images (see Readme.md) and some
major clean up on the directory structure of the repository. The major changes
include

• Pandoc 2.17 is used in all docker images.
• convert folder has been moved to tools/convert to decouple the build
  from the markdown source. The upgrade makes it possible to use docker images
  technical-markdown:<version> to build your own sources without the need to
  have the tools folder also in your own repository. Also a different source
  layout is possible by adapting the build.gradle.kts. If you want to
  experiment in your fork or own repository with different layouts/styling and
  pandoc conversions you can use the technical-markdown:<version>-minimal
  image which does not include the tools folder. Any feature improvement which
  could be of general interest is highly welcome.