doc: use plantweb as fallback of sphinx-ditaa

doc: use plantweb as fallback of sphinx-ditaa

RTD does not support installing system packages, the only ways to install
dependencies are setuptools and pip. while ditaa is a tool written in
Java. so we need to find a native python tool allowing us to render ditaa
images. plantweb is able to the web service for rendering the ditaa
diagram. so let's use it as a fallback if "ditaa" is not around.

also start a new line after the directive, otherwise planweb server will
return 500 at seeing the diagram.

Signed-off-by: Kefu Chai <kchai@redhat.com>