Replace all downloadable file images currently in main with access date images

[jaclyn-taroni]

⚠️ Stacked on #180 – see that PR for the sentence included at the beginning of the downloadable files section.

Here, I'm addressing the part of #171 concerned with download illustrations already in main: project, sample, multiplex, and spatial. Ultimately, I've chosen to get this into main via the documentation-improvements branch, which contrasts what I wrote on the issue initially (written before our discussion on Friday). We will eventually merge documentation-improvements -> main and then main -> development, so I figured there was no harm in this approach.

The most important thing to check is that I didn't save an image under the wrong name!

[dvenprasad]

Here is the updated multiplexed download folder