Document: feat: introduce rwadmin user for cloud control plane

[neverchanje]

This issue tracks the documentation update needed for the merged PR #22327.

Source PR URL: risingwavelabs/risingwave#22327
Source PR Merged At: 2025-06-24T09:35:02Z

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.