(MAINT) Update TOC for schema articles

[michaeltlombardi]

PR Summary

Prior to this change, the overview and the article explaining the schema URIs for DSC were missing from the TOC. This change adds entries for those articles.

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 39689f9:

✅ Validation status: passed

File	Status	Preview URL	Details
dsc/docs-conceptual/dsc-3.0/toc.yml	✅Succeeded	View (dsc-3.0)

For more details, please refer to the build report.

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[sdwheeler]

LGTM