BD-3739: Add 2 Canvas templates use cases

[lydia-xie]

Why are you making this change? (required)

Adds new Canvas template use case articles for:

• Onboarding with preferences survey
• Email sign-up with double opt-in

Related PRs, issues, or features (optional)

• BD-3739

Feature release date (optional)

• N/A

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[rachel-feinberg]

Lgtm! Left a few comments!