Update documentation about Kubernetes executor authentication

What does this MR do?

Followup of gitlab-runner!2048 (merged). It updates the docs on GitLab Runner Kubernetes Executor supporting $DOCKER_CONFIG_AUTH. It was added with runner 13.1.

Also points out that -n is most probably needed when encoding a password for use with it. Following up on: gitlab-runner!2048 (comment 368917338)

Related issues

gitlab-runner#2673 (closed)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have developer access or higher (for example, GitLab team members or Core Team members)
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group label, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
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  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add GitLab's version history note(s).
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• 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

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

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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3. Maintainer

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