Add conditional router tutorial

[bilgeyucel]

Fixes #279

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[TuanaCelik]

Do we nees ro have 2 prompt templates? I think we could make it so that the one promopt template can have a {% if documents_from_web %} block which gets filled in if there's a route from go_to_websearch but maybe I'm off here..

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[bilgeyucel]

It's totally doable as an alternative solution where we create a loop in the pipeline. I didn't want to explain the loop in this tutorial but rather wanted to explain branches

[bilgeyucel]

After rechecking the pipeline, I don't think using a Multiplexer adds any value here.
Current run():

pipe.run({"prompt_builder": {"query": query, "documents": documents}, "router": {"query": query}})

run() with a Multiplexer:

pipe.run({"prompt_builder": {"documents": documents}, "multiplexer": {"query": query}})

Now that we've decided not to switch to ChatGenerators, there is nothing to change here. @TuanaCelik please take one last look when you can 🙌