docs(ui): replace removed useAssistant example link

[Kage18]

Summary

• replace the removed vercel-labs assistant example link in the AI SDK v5 migration guide
• point users to maintained internal chatbot and useChat docs instead

Fixes #7571.

Kage usage

• Used Kage scoped to packages/ai to capture the migration-guide decision for future agents.
• Local Kage metrics after refresh: 436 files, 7,411 symbols, 8,863 calls, 2,068 tests, 100% indexed coverage, 3 memory packets, 53 memory graph entities, 55 evidence-backed edges, 0 stale packets.

Tests

• git diff --check

Docs-only change.

[Kage18]

Kage usage for this PR:

This PR fixed #7571 by replacing a removed useAssistant example link in the AI SDK v5 migration guide with maintained docs/examples.

How Kage helped:

• Scoped recall to the migration-guide docs area and avoided changing unrelated UI package code.
• Identified the exact target doc: content/docs/08-migration-guides/26-migration-guide-5-0.mdx.
• Helped frame the change as link maintenance and migration guidance, not an API behavior change.
• Captured the reusable docs decision: removed external examples should be replaced with maintained internal references where possible.
• Captured the lightweight verification path: git diff --check.

Why this matters for future sessions:
A future agent updating migration docs can recall why this link was changed and prefer maintained AI SDK docs over stale external example repos.

Full contribution summary and Kage metrics: kage-core/Kage#8