[Realtime] Fix broken use cases link on overview page

[swapnilmadavi]

Summary

The "Use cases" card on the Cloudflare Realtime overview page (/realtime/) linked to
/realtime/realtimekit/introduction#use-cases, a page that doesn't exist. This caused a 404 when clicked.

Updated the link to /realtime/realtimekit/#build-with-realtimekit, which points to the existing
"Build with RealtimeKit" section on the RealtimeKit overview page.

Fixes #32328

[cloudflare-docs-bot]

Review

✅ No issues found in commit 5dbb2e9.

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[korinne]

LGTM