Migration guide: `azure_data_cosmos` (from azure_data_cosmos ≤0.21.0 / azure_sdk_cosmos)

Part of #4228

## Migration guide: `azure_data_cosmos`

### Official Crate

- **Name:** [`azure_data_cosmos`](https://crates.io/crates/azure_data_cosmos)
- **Current version:** 0.32.0
- **Service:** Azure Cosmos DB

### Unofficial Predecessor(s)

| Predecessor | Version(s) | Relationship | crates.io |
|---|---|---|---|
| `azure_data_cosmos` v0.1.0–0.21.0 | Era 2 (Jan 2022 – Oct 2024) | **Same-crate older version** — complete rewrite at v0.22.0 (Feb 2025) | [crates.io](https://crates.io/crates/azure_data_cosmos) |
| `azure_sdk_cosmos` v0.100.3 | Era 1 (MindFlavor, 2017–2019) | **Different crate** — community-era Cosmos DB client, 44K downloads | [crates.io](https://crates.io/crates/azure_sdk_cosmos) |

> **Note:** Cosmos DB GA is targeting end-of-June 2026, not the April 2026 wave.

### Migration Guide Should Cover

- Complete API rewrite at v0.22.0+ (new client, database, container hierarchy)
- New query builder and response models
- Cross-partition query improvements
- Authentication changes (key-based and Entra ID)
- Error handling redesign
- Migration path for `azure_sdk_cosmos` users (start from scratch)
- Cargo.toml dependency changes

### Acceptance Criteria

- [ ] Migration guide published in repo docs (e.g., `docs/migration/azure_data_cosmos.md`)
- [ ] Code samples for key scenarios: creating a client, CRUD operations, querying documents, cross-partition queries
- [ ] FAQ section covering common migration pitfalls
- [ ] Linked from the `azure_data_cosmos` crate README
- [ ] Reviewed by SDK team

Start with the current `azure_data_cosmos` API and the migration scope in `docs/migration/azure_data_cosmos.md`; compare it with the listed predecessor versions and the crate README. Document client, database, container, CRUD, query, cross-partition, authentication, error-handling, and Cargo.toml changes, then add the FAQ, link the guide from the README, and prepare it for SDK team review.

Written by the indexing model from the issue text.