Add docs for running shfmt on shell scripts

What does this MR do?

Now that this comment has been addressed and an official alpine-based Docker image is available, we can remove replace the note in the shell scripting guide docs with an example of how to automatically add a shfmt job to the CI/CD pipeline.

Related issues

No related issue

Testing

Tested in this pipeline

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• Apply the documentation label, plus:
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  • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
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• Assign the designated Technical Writer.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add GitLab's version history note(s).
• Add the product tier badge.
• Add/update the feature flag section.
• If you're changing document headings, search doc/*, app/views/*, and ee/app/views/* for old headings replacing with the new ones to avoid broken anchors.

Review checklist

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1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

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