Improve CI variables Syntax of env variables section

What does this MR do?

• Add seperate sections to Syntax of environment variables in job scripts section
• List additional PowerShell variable option
• Add PowerShell variables display details
• Put Batch details last since it's depreciated
• Fix gitlab-runner#4115 (comment 157692820) by documenting need to sometimes surround vars with quotes

Related issues

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)
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When applicable:

• Update the permissions table.
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• 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

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