Docs: add samples for creating KafkaConsumerActor for typed actor systems

[ennru]

The Alpakka Kafka documentation should show how the classic KafkaConsumerActor is created when the user has a typed actor system.

[johanandren]

Is there something more to it than adapting the system with .toClassic ?

[ennru]

Not really, but still relevant to show, I believe.
The work is more to put that into compiling code for Alpakka Kafka docs.

[johanandren]

If you update to require Akka 2.5.27 you can have the methods take an ClassicActorSystemProvider and that will make it possible to pass both typed and classic actor systems. (Or is it already 2.5.26 perhaps)

[johanandren]

No, sorry, we never back ported that, scratch last comment.

[ennru]

Now that we have a module which is compiled only with Akka 2.6, this is much easier to add.

[seglo]

It's a bit of a franken-module though. It's not included in the rest of the build so that it doesn't pull up Akka 2.6 deps inadvertently. Once 2.5 support is dropped it will be reintegrated, but for now the only place to add such a code example would be in the module itself, which may be fine, in the meantime.

[seglo]

Deferring until 2.1.0

[ennru]

Once #1209 is merged, we should review the doc examples and consider primarily showing the new actors API in the docs.

[seglo]

Implemented in #1370.