fix: birdwatcher template and document issues

[MojtabaHs]

All changes are fully backward compatible.

So we can add [no important files changed] to merge-commit message.

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[meatball133]

The introduction docs for concept exercises also execist under concepts/concept/about.md and introduction,md. In this case they are located under the concept for-loops. Both of which has to be updated.

For the Swift track to keep things simple have I made so the introduction docs for the exercise is the same as for the concept (unless an exercise have many concepts). The about file might differ and might have some extra headings the introduction one don't have.

[MojtabaHs]

I have fixed them as requested. I think we should make a single source of truth for all identical documents. Maybe using links 🤷🏻‍♂️

[meatball133]

There is this tool which is part of configlet (the cross-track Exercism tool): https://exercism.org/docs/building/tracks/concept-exercises#h-file-docs-introduction-md-tpl. However, it only allows the introduction to be templated. Not the about file.

Alternatively, you can develop a custom solution (when becoming maintainer). I have done such a thing on the main track I maintain: https://github.com/exercism/crystal/tree/main/document-generator, I haven't worked on it for a while but it does work.