akka streams docs: graphs

WIP on graphs parts of docs.

refs https://github.com/akka/akka/issues/16549
Need help on that one then, what about:

```
+A :class:`PartialFlowGraphs` is defined as a :class:`FlowGraph` which contains so called "undefined elements",
```

should I update to "undefined stages"? In general had this problem in many places, that we don't have a common name for the building blocks.