## Stack context
This is the second PR in the V5 documentation stack, on top of the versioned API reference.
## What and why
Add an agent-oriented V3/V4 to V5 migration guide covering document ingestion and updates, content management, search, profiles, forgetting, namespaces, organization settings, typed filters, and rollout verification. Shared snippets keep the comprehensive guide and focused topic pages consistent.
```mermaid
flowchart LR
Legacy[Legacy integration inventory] --> Mapping[Domain migration guidance]
Mapping --> V5[V5 requests and response readers]
V5 --> Verify[Side-by-side verification and rollout]
```
## Validation
- Verified all 11 migration navigation entries resolve to authored pages.
- Verified all imports across 23 migration and snippet files resolve.
- `git diff --check` passed.
- Mintlify build validation passed against the local generated V5 OpenAPI snapshot.
## Impact
Documentation only. The guide explicitly covers changed defaults, removed operations, partial failures, namespace isolation, and rollback-oriented side-by-side testing.
- Published the v2.0.0 docs and a 1.4 → 2.0 migration guide so existing users have a clear upgrade path.
- Updated the four integration pages (AI SDK, OpenAI, Mastra, VoltAgent) to reflect v2 defaults and link to the migration guide.
- Added a short explainer on the two required fields (containerTag, customId) so new users aren't blocked at first integration.
