Add merged images to docs

[sjspielman]

Closes #218 Closes #245

This PR adds the merged images into docs.

While I was here, I had another overall look at the file. I ended up changing one of the things that had previously been requested in review here https://github.com/AlexsLemonade/scpca-docs/commit/758dc1d536bc9ba89421daf12010a5bacd2355aa - I moved the bulk files description back into the metadata section. This is subject to discussion here!!

I made this change for two reasons:

• Leaving the bulk text where it was effectively nested it inside a "section" for the anndata cite download image, and I think this would not work well in the actual docs if sections were collapsed
• In the other spots where we mention mention bulk metadata in this document, it tends to follow a description (or at least a mention) of the single cell metadata: https://github.com/AlexsLemonade/scpca-docs/blob/df204a850d7c3c12124bc3d2c89250172d21a73a/docs/download_files.md?plain=1#L24-L28

https://github.com/AlexsLemonade/scpca-docs/blob/df204a850d7c3c12124bc3d2c89250172d21a73a/docs/download_files.md?plain=1#L122-L123 ^ I also grabbed the second sentence here and added it into the merged bulk description.

Let me know if you disagree with this change!