Refine `/api/exporter` response

[slint]

There are a couple of readability enhancements that can be done to the /api/exporter endpoint's response:

• is_head -> is_latest
• links.self_head -> links.self_latest or links.latest?
• ...

Any changes to this would also require updating the developer docs

[ptamarit]

I had used the name is_head based the field in ObjectVersionSchema, but indeed, it's probably not the best name.

[slint]

I was also thinking about somehow "merging" the metadata files with their deleted records CSV, since now they're actually kind of disconnected and we might have the latest 3 snapshots of the CSV (but not the associated datasets).