docs-rst: convert pipeline to SVG format

The pipeline image was produced from some dot file that has
long missed. Create a pipeline.dot with the graph and convert
it to SVG. As we're planning to add future support for graphviz
graphics, also store the .dot file on the tree, as this will
make easier when we add such Sphinx extension.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>