[doc] Add R tab to parameter tuning tutorial #12501

Summary

Add R code tabs alongside the existing Python examples in the PARAM_TUNING tutorial (doc/tutorials/param_tuning.rst).

The current Reducing Memory Usage section contains several Python examples for train/test splitting, dropping dataframe columns, checking/converting array types, and avoiding unnecessary memory copies. This issue proposes adding equivalent R examples using the existing .. tabs:: / .. code-tab:: structure.

The R examples should:

• provide equivalent R code where applicable
• avoid introducing unnecessary package dependencies
• preserve the intent of the existing memory-usage recommendations
• keep the Python examples unchanged

This is part of #11413.