[doc] Add R tab to parameter tuning tutorial

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](https://github.com/dmlc/xgboost/issues/11413).

Read doc/tutorials/param_tuning.rst, starting with the Reducing Memory Usage section and its existing .. tabs:: / .. code-tab:: structure. Add equivalent R tabs for the listed train/test splitting, dataframe-column, array-type, and memory-copy examples without changing the Python examples or adding unnecessary dependencies. Done when each applicable recommendation has a matching R example that preserves the tutorial's intent.

Written by the indexing model from the issue text.