[doc] Add R tab to DART tutorial

[medisean]

Summary

• present the existing Python DART sample in a language tab
• add an equivalent R sample using xgb.DMatrix() and xgb.train()
• keep the parameters and training flow aligned across both languages

Part of #11413.

Testing

• git diff --check
• verified the R parameter names and xgb.train() signature against the current R package source
• checked the RST tab structure and end-of-file newline