RFR: JDK-8232639: Change module graph images to use SVG instead of PNG format.

[Jonathan Gibbons]

Please review a small change in the build, to use .svg files instead of 
.png files,
for the generated module graph images.

The underlying generator, GraphViz, already supports .svg output.

In the proposed patch, the filename is changed in the generator tool,
and all references to "png" in make/Docs.gmk are converted to "svg".

-- Jon


JBS: https://bugs.openjdk.java.net/browse/JDK-8232639
Webrev: http://cr.openjdk.java.net/~jjg/8232639/webrev.00/index.html
Sample API docs: 
http://cr.openjdk.java.net/~jjg/8232639/api.00/api/index.html