docs: don't require 'dia' to build the docs

These diagrams are not expected to change much, so just include the generated
PNGs in the repository, and remove 'dia' from the prerequisites to build the
documentation.

data/Makefile.am

@@ -57,6 +57,13 @@ logos = \
 	ModemManager-logo-wide-text.svg ModemManager-logo-wide-text.png
 
 
+# Diagrams
+diagrams = \
+	ModemManager-states.png \
+	ModemManager-interface-initialization-sequence.png \
+	ModemManager-interface-initialization-sequence-subclassed.png
+
+
 # Polkit
 dist_polkit_policy_in_files = org.freedesktop.ModemManager1.policy.in
 if WITH_POLKIT
@@ -90,4 +97,5 @@ EXTRA_DIST = \
 	$(dbusservice_file_nopolkit) \
 	$(icon_DATA) \
 	$(dist_polkit_policy_in_files) \
-	$(logos)
+	$(logos) \
+	$(diagrams)