Added intro paragraph

[mikefrobbins]

PR Summary

Fixes AB#305803 - Added intro paragraph.

PR Checklist

• Descriptive Title: This PR's title is a synopsis of the changes it proposes.
• Summary: This PR's summary describes the scope and intent of the change.
• Contributor's Guide: I have read the contributors guide.
• Style: This PR adheres to the style guide.

[learn-build-service-prod]

Learn Build status updates of commit 163c10c:

⚠️ Validation status: warnings

File	Status	Preview URL	Details
docs-conceptual/azps-12.3.0/upcoming-breaking-changes.md	⚠️Warning	View (azps-12.3.0)	Details

docs-conceptual/azps-12.3.0/upcoming-breaking-changes.md

• Line 259, Column 143: [Warning: disallowed-html-tag - See documentation] HTML tag 'String' isn't allowed. Replace it with approved Markdown or escape the brackets if the content is a placeholder.

For more details, please refer to the build report.

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