Improve empty states when there is no input available

What does this MR do and why?

Update the empty state to add a link to the documentation and hide the input selector buttons when there's no inputs available for the configuration.

References

Screenshots or screen recordings

Before After
Screenshot_2025-06-18_at_21.45.26 Screenshot_2025-06-18_at_21.44.40

How to set up and validate locally

  1. Navigate to Build -> New pipeline and verify the pipeline inputs empty state

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #549114 (closed)

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