doc: Generate PlantUML diagrams automatically

doc: Generate PlantUML diagrams automatically

Currently we have some pre-rendered versions of certain diagrams
in SVG format. These diagrams have corresponding PlantUML source
that can be rendered automatically as part of the documentation
build, removing the need for any intermediate files.

This patch adds the Sphinx "plantuml" extension, replaces
references to the pre-rendered SVG files within the documents,
and finally removes the SVG files and helper script.

New requirements for building the docs are the
"sphinxcontrib-plantuml" Python module (added to the pip
requirements.txt file) and the Graphviz package (provides the
"dot" binary) which is in the Ubuntu package repositories.

Change-Id: I24b52ee40ff79676212ed7cff350294945f1b50d
Signed-off-by: Paul Beesley <paul.beesley@arm.com>