add index page to use-cases

[marybelzer]

No description provided.

[netlify]

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[sourcegraph-bot]

Notifying subscribers in CODENOTIFY files for diff 92667e5...bb90808.

Notify	File(s)
@christinaforney	content/departments/product-engineering/product/index.md content/strategy-goals/strategy/index.md content/strategy-goals/strategy/use-cases/code-health.md content/strategy-goals/strategy/use-cases/code-reuse.md content/strategy-goals/strategy/use-cases/dev-onboarding.md content/strategy-goals/strategy/use-cases/fixing-security-vulnerabilities.md content/strategy-goals/strategy/use-cases/incident-response.md content/strategy-goals/strategy/use-cases/index.md content/strategy-goals/strategy/working-with-use-cases.md
@greggstone	content/departments/sales/index.md
@sourcegraph/marketing	content/departments/marketing/index.md

[marybelzer]

Fixes #2066