Optimize SVG illustration file sizes

I optimized some of the illustration images used on the top page using svgo.
Some illustration files were already optimized, but there were a few files where optimization caused rendering issues in certain browsers, so I had temporarily decided not to optimize them. This time, I attempted to optimize and reduce the file sizes of those files as well.
While some files didn't change much, the total reduction was -29.1 KiB, approximately 14.6%.

Author: taea