Add warnings to stop using blockdiag as it is orphaned

See thesofproject#472 for details.

Signed-off-by: Marc Herbert <[email protected]>

architectures/firmware/sof-zephyr/mpp_layer/images/mpp_scheduling/edf_scheduling.diag

@@ -1,3 +1,6 @@
+// FIXME: blockdiag is orphaned and not compatible with Pillow anymore:
+// https://github.com/blockdiag/blockdiag/pull/171
+
 blockdiag edf_scheduling {
 
 	node_width = 250;

conf.py

@@ -30,6 +30,10 @@
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
+
+
+# FIXME: blockdiag is orphaned and not compatible with Pillow anymore:
+# https://github.com/thesofproject/sof-docs/issues/472
 extensions = ['breathe', 'sphinx.ext.graphviz', 'sphinxcontrib.plantuml',
               'sphinx.ext.todo', 'sphinx.ext.extlinks', 'sphinxcontrib.blockdiag'
 ]