BD-2979: Create Catalog Triggers subsection and relocate articles

[bre-fitzgerald]

Summary

Creates a subsection under Catalogs for Catalog triggers, and relocates the Back in stock and Price drop notification articles to that subsection. This change is to prepare for the release of more catalog triggers in the future by preventing the main catalog nac section from being too crowded.

Related PRs, issues, or features (optional)

• BD-2979

Feature release date (optional)

• N/A

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[lydia-xie]

lgtm!