Explain how to create user worker run for Arkindex CLI on demo

Refs https://redmine.teklia.com/issues/9706

When a user follows the tutorial and tries to load data through the teklia-scrapers tool, they need to create a worker run.

We decided to create a shared worker so they do not have to create their own worker with dummy DOcker images. You need to replace the --worker-run-id explanation (especially "Refer to this page to create your own worker run.") by a note/paragraph explaining that on demo.arkindex.org, due to limited access rights for freely signed up users, they need to use the "Arkindex CLI" worker and its version to create a worker run (and NOT try to create one).