Add change alerts to Document Harvester

[anjackson]

There are some use-cases for alerts that are specific to the Document Harvester:

• "Page change" alerts if name of body that issues the documents change?
• Or the document pack changes. e.g. new patent surveys to describe? (i.e. a page with links to each survey).
• The name of the organization changes, on the same link, do what?
• Or a domain dies then what?

It's possible that this would work via a simple interface like ukwa/ukwa-access-api#28, but with some bundling by Target. And ideally this would need some kind of change visualization, as per GLAM Workbench.

This gets challenging rather quickly, so would perhaps need a dedicated fund.