Update Multiplexer tutorial with the BranchJoiner

[bilgeyucel]

Two changes are needed to be made here:

• This component's main job is to merge multiple branches. Simplifying pipeline runs is not a valid use case for it, and we need to come up with a new example
• The component will be renamed as BranchJoiner, so the name should change

[bilgeyucel]

Tutorial: https://haystack.deepset.ai/tutorials/37_simplifying_pipeline_inputs_with_multiplexer