Migrate documentation from gitbook to honkit

[bruderol]

gitbook is no longer maintained. Honkit is its fork that seems to be actively maintained.
See https://github.com/honkit/honkit

Should be easy to migrate and could fix our current workaround for uml diagram deployment that is ugly.

See very strange fix in .circleci/config.yml:

            echo "copy missing assets - over assets in _book - there seems to be a problem with generated uml diagrams otherwise ..."
            cp -rf assets _book

Which was only needed with current gitbook uml plugin (needs to be updated as well!!) in newer node version.