Document: feat: support schedule/reschedule resource group

This issue tracks the documentation update needed for the merged PR #19955.

Source PR URL: https://github.com/risingwavelabs/risingwave/pull/19955
Source PR Merged At: 2025-01-17T07:49:28Z

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.