Docs: Create separate CSS files, do not embed into HTML

Docs: Create separate CSS files, do not embed into HTML

TODO: There still is one set of CSS files per subdirectory due to the
way the docs directories are organised. It would be better to have them
just once. But to achieve that, the ADOC-to-HTML conversion would have
to be changed, e.g. using one central directory for images and
reorganisation of the overall directory structure and build options.
See https://github.com/asciidoctor/asciidoctor-maven-plugin/issues/729.

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>