Alt text on decorative images on timeline is useless

[jchristgit]

As pointed out in #1363:

Python logos and other icons in the center line of the timeline are read as “logo image” and other generic names.

While descriptive text is imperative to describe images to those who can’t see them, these images are purely decorative and add no value to someone not seeing the page. Use alt=”” instead so the experience with a screen reader is less cluttered.