document feature of accessing individual files in artifacts

What does this MR do?

This MR updates the artifact download docs to mention the feature of downloading specific files inside a build (instead of the whole zip file).

Are there points in the code the reviewer needs to double check?

There may be other magic URLs like this that should also be documented - I haven't looked at the source code to see if there was anything else missing. I was only told on IRC this is how the pelican project was doing it.

Why was this MR needed?

Documentations. It is sweet and tasty. Plus, I am likely to forget this exists and will get upset for no reason at gitlab again. ;)

Screenshots (if relevant)

Not really...

Does this MR meet the acceptance criteria?

• Changelog entry added - not necessary?
• Documentation created/updated - this is documentation, obviously someone didn't follow those docs when that feature was introduced :p
• API support added - N/A
• Tests - N/A - no tests for docs, right?
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Not that I know of, but I didn't review the issue queue.