docs: Pipeline.inputs()

[TuanaCelik]

Once a pipeline is created, it's difficult for users to know how they should run the pipeline. We have quite a useful utility function for pipelines which is .inputs() which lists all the expected/required inputs for the components.

We should use this in our docs heavily imo. This function is hardly visible, and we don't really provide any help on how a pipeline should be run other than trial and errors. Once a pipeline fails, the error us quite handy. And if a user knows to ues .show() that is also handy. But I think we should add this everywhere as something that a user colud make use of while creating/running pipelines.

1. In the pipelines doc
2. In all component docs where we have example pipelines..

[aantti]

I'd love that. Specifically, btw, struggled recently with OpenAITextEmbedder trying to figure out the correct parameter ('text').

[dfokina]

I have added the following paragraph to the docs that should be well-searchable for pipeline inputs or component inputs: https://docs.haystack.deepset.ai/docs/creating-pipelines#pipeline-inputs