Using workshop id/slug to set up etherpad

In the [Working with your Team section](https://carpentries.github.io/instructor-training/21-management.html), we say they can create an etherpad, "preferably using the same workshop id used when creating the workshop website." Should we remove the word "preferably" to make it more emphatic that they **should** use the same id as the workshop website? It makes it much easier to find things later.

Also, along those lines, here we say "workshop id" but everywhere else we call it "workshop slug." Should we call it "slug" here as well, to be clear we are always talking about the same thing?

Open the Working with your Team section in 21-management.html and review the wording around creating an Etherpad. Replace the tentative wording with a clear recommendation to use the workshop slug, then check the surrounding documentation for consistent use of “workshop slug” instead of “workshop id.”

Written by the indexing model from the issue text.