docs: add OpenAIGenerator to OpenAIEmbedders pipeline examples

[dfokina]

[julian-risch]

@dfokina Why do we need to add the Generator to the embedder pipeline example? For example, this one here is an example of an indexing pipeline that works fine without a generator: https://docs.haystack.deepset.ai/docs/openaitextembedder#in-a-pipeline

[dfokina]

@julian-risch I created this issue based on a request from Andrey: https://deepset-ai.slack.com/archives/C035R9A65NJ/p1724229905783589

[julian-risch]

Thanks, I had a look now. I don't think that the OpenAIEmbedder page and it's pipeline example is the right place to show how to use OpenAIGenerator. Let's discuss alternative solutions first. So far, we have functional yet minimal examples and my concern is that we would give up the "minimal" part here.