Add dependency graphs to the documentation

[GPMueller]

It would be helpful for quick understanding of dependencies in general and specifically how clang-build deals with them to have dependency graphs for the examples in the documentation. They are easily generated with viz-js:

This might also motivate us to create better graphs that are easier to read...
Note: it would be nice to have a graph property rankdir="BT" in the dotfile output, which would make the top level nodes appear at the top of the image.