Move sequence diagrams to PlantUML SVG.

The plantuml source used to generate the diagrams is embedded in the SVG
resources and can be modified and regenerated (by hand, at this time).

Remove annotation on Reader.

website/src/documentation/io/developing-io-overview.md

@@ -107,9 +107,9 @@ Here is a sequence diagram that shows the lifecycle of the Source during
  information to IO developers such as the constraints that 
  apply to the objects or particular cases such as streaming mode.
 
-
-  ![This is a sequence diagram that shows the lifecycle of the Source](
-    {{ "/images/source-sequence-diagram.png" | prepend: site.baseurl }})
+ <!-- The source for the sequence diagram can be found in the the SVG resource. -->
+![This is a sequence diagram that shows the lifecycle of the Source](
+    {{ "/images/source-sequence-diagram.svg" | prepend: site.baseurl }})
 
 ### Using ParDo and GroupByKey
 

website/src/documentation/programming-guide.md

@@ -809,9 +809,9 @@ Here is a sequence diagram that shows the lifecycle of the DoFn during
  apply to the objects or particular cases such as failover or 
  instance reuse. They also give instanciation use cases.
 
- 
+<!-- The source for the sequence diagram can be found in the the SVG resource. -->
 ![This is a sequence diagram that shows the lifecycle of the DoFn](
-  {{ "/images/dofn-sequence-diagram.png" | prepend: site.baseurl }})
+  {{ "/images/dofn-sequence-diagram.svg" | prepend: site.baseurl }})
 
 #### 4.2.2. GroupByKey {#groupbykey}