party-robot: change taught concepts

[ErikSchierboom]

The party-robot exercise currently is designed to teach the strings concept, but this will be done by the Poetry Door Club policy exercise that is to be added. As such, the goal of this exercise changes to: teach the string-formatting concept (and the package-import concept.

The strings concept should be renamed to string-formatting: https://github.com/exercism/go/blob/main/config.json#L64

[junedev]

@bobtfish I think there is a part missing here. The the exercise introduction and the concept introduction and about file still contain a section that explain strings in general. That section should be removed as this part will be covered by another exercise.

You can paste the text as a comment into this issue #1613 so the person creating the string exercise/concept can use it.

Sorry that I did not notice this in the review earlier but it was a bit hard to understand which PR was supposed to cover which issue. You can help with that by writing "resolves #xyz" (where xyz is the issue number) into the PR description. GitHub will then automatically create a link. See https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword