[ImgBot] Optimize images (#51)

*Total -- 647.25kb -> 458.09kb (29.22%)

/doc/source/images/als-diagram.png -- 246.24kb -> 146.23kb (40.61%)
/doc/source/images/download-apache-spark.png -- 91.03kb -> 54.19kb (40.47%)
/doc/source/images/architecture.png -- 123.60kb -> 96.61kb (21.84%)
/doc/source/images/launch-notebook.png -- 33.38kb -> 28.02kb (16.06%)
/doc/source/images/notebook-run-cells.png -- 153.00kb -> 133.04kb (13.04%)

Author: imgbot[bot]