Document: feat: Add parallelism configuration for each streaming_job type

This issue tracks the documentation update needed for the merged PR #21366.

Source PR URL: https://github.com/risingwavelabs/risingwave/pull/21366
Source PR Merged At: 2025-04-15T09:32:28Z

If it is a major improvement that deserves a new page or a new section in the documentation, please check if we should label it as an experiment feature.