A hack to fix broken search bar in doc (#2583)

Current version of xgboost.readthedocs.io has a broken search box.
Enabling themes on ReadTheDocs is known to break the search function, as
reported in
[this document](readthedocs/readthedocs.org#1487). To get
around the bug, we replace the `searchtools.js` file with our custom version.

Author: hcho3