docs: Drop sphinxcontrib.{actdiag,blockdiag}, use sphinx.ext.graphviz
sphinxcontrib.actdiag and sphinxcontrib.blockdiag have not been
updated since 2021 and both have a dependency on a version of Pillow
that has vulnerabilities. So drop the use of sphinxcontrib.actdiag
and sphinxcontrib.blockdiag, and use sphinx.ext.graphviz instead.
.pipeline/blubber.yaml: Add the graphviz package which is now
needed to build docs.