Exclude _images and _sources that are generated by Sphinx in Jekyll build

For _images directory, after SPARK-31851, now we added some images to use within the pages built by Sphinx. It copies and images into _images directory. Later when Jeykill builds, the underscore directories are ignored by default which ends up with missing image in the main doc. I attached the image in the JIRA.

For _sources directory, see https://github.com/sphinx-contrib/sphinx-pretty-searchresults#source-links.