Docs: Highlight/explain benefits of artifacts with custom keys

[serinamarie]

First check

• I added a descriptive title to this issue.
• I used GitHub search to find a similar request and didn't find it 😇

Describe the issue

Users create artifacts without knowing what a key is; then they expect artifacts to show up on their artifacts page, when instead they are associated with only the flow or task run they were created from (and so can only be viewed there).

Describe the proposed change

We should highlight the benefits of keyed artifacts. @znicholasbrown will also be enhancing the UI to show unkeyed artifacts down the road: PrefectHQ/prefect-ui-library#1436

Add UI screenshots

Additional context

No response