Add a merged objects section to Downloadable files

[jaclyn-taroni]

We need to describe what merged object downloads will be like in the Downloadable files section and include illustrations (SCE, AnnData, and CITE-seq).

[sjspielman]

From slack conversation with @dvenprasad:

For download files: If we decide to make merged objects a separate page, then we should tuck the downloadable files for merged objects under a Merged objects section on downloadable files page so we’re being consistent with that distinction.

(Edit: note in #219 we indeed decided to make merged objects a separate page)

But where should this section go?

We can just insert that section after AnnData - and show both SCE merged and AnnData merged.

[sjspielman]

Noting I'm assigning myself here for the description part, and @dvenprasad for the illustration part. We can also break this up into two issues if that's preferred!

[sjspielman]

Ah I see #218 was already broken out for issues! Congratulations @dvenprasad, I'm removing you here and instead blocking this with #218.

[dvenprasad]

@sjspielman The illustrations are done. Let me know if anything is missing on #218

[sjspielman]

Closed by #234