A hack to fix broken search bar in doc

[hcho3]

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. To get around the bug, we replace the searchtools.js file with our custom version.

How to test the custom script before merging this PR:

• Download searchtools.js to your home folder.
• Run python -m SimpleHTTPServer.
• Open the web browser and browse to http://localhost:8000/searchtools.js
  Make sure that you can access the script using this address.
• Download the Google Chrome extension Resource Override.
  This will let you subtitute the custom script for the hosted script.
• Set following the replacement rule in Resource Override:

   https://xgboost.readthedocs.io/en/latest/_static/searchtools.js
=> http://localhost:8000/searchtools.js

• Browse to https://xgboost.readthedocs.io/en/latest/ and try the search box.
  Google Chrome will warn you about "loading scripts from unauthenticated sources." Click "Load unsafe scripts."

• The search function should work with the custom script loaded.

• Once done, make sure to delete the Google Chrome extension and shut down SimpleHTTPServer.

[hcho3]

@ysh329 This might be an interest to you.

[ysh329]

@hcho3 Thanks! 👍

[codecov-io]

Codecov Report

Merging #2583 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2583      +/-   ##
==========================================
+ Coverage   35.08%   35.11%   +0.02%     
==========================================
  Files          79       79              
  Lines        6971     6971              
  Branches      680      680              
==========================================
+ Hits         2446     2448       +2     
  Misses       4423     4423              
+ Partials      102      100       -2

Impacted Files	Coverage Δ
src/data/sparse_page_dmatrix.cc	87.97% <0%> (+1.09%)	⬆️

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[terrytangyuan]

Thanks!