Migration guide: `azure_messaging_eventhubs` (from azeventhubs)

Part of #4228

## Migration guide: `azure_messaging_eventhubs`

### Official Crate

- **Name:** [`azure_messaging_eventhubs`](https://crates.io/crates/azure_messaging_eventhubs)
- **Current version:** 0.13.0
- **Service:** Azure Event Hubs

### Unofficial Predecessor(s)

| Predecessor | Version(s) | Relationship | crates.io |
|---|---|---|---|
| `azeventhubs` v0.20.0 | Community (minghuaw, 211K downloads) | **Different crate** — unofficial AMQP 1.0 Event Hubs client | [crates.io](https://crates.io/crates/azeventhubs) / [repo](https://github.com/minghuaw/azeventhubs) |

> **Note:** There was no era 2 Azure-published Event Hubs crate. The community `azeventhubs` crate by [minghuaw](https://github.com/minghuaw) was the primary Rust Event Hubs client (211K downloads).

### Migration Guide Should Cover

- Differences between community `azeventhubs` and official `azure_messaging_eventhubs`
- `EventProcessor` for scalable event consumption
- Geo-replication support
- Auto-reconnect and resilience improvements
- Dedicated `EventHubError` types
- Companion crate: [`azure_messaging_eventhubs_checkpointstore_blob`](https://crates.io/crates/azure_messaging_eventhubs_checkpointstore_blob) for blob-backed checkpointing
- AMQP transport layer via `azure_core_amqp`
- Cargo.toml dependency changes

### Acceptance Criteria

- [ ] Migration guide published in repo docs (e.g., `docs/migration/azure_messaging_eventhubs.md`)
- [ ] Code samples for key scenarios: producing events, consuming events with EventProcessor, checkpoint store setup, error handling
- [ ] Section covering `azure_messaging_eventhubs_checkpointstore_blob` companion crate
- [ ] FAQ section covering common migration pitfalls (especially for `azeventhubs` users)
- [ ] Linked from the `azure_messaging_eventhubs` crate README
- [ ] Reviewed by SDK team