Migration guide: `azure_storage_blob` (from azure_storage_blobs / azure_sdk_storage_blob)

Part of #4228

## Migration guide: `azure_storage_blob`

### Official Crate

- **Name:** [`azure_storage_blob`](https://crates.io/crates/azure_storage_blob)
- **Current version:** 0.11.0
- **Service:** Azure Blob Storage

### Unofficial Predecessor(s)

| Predecessor | Version(s) | Relationship | crates.io |
|---|---|---|---|
| `azure_storage_blobs` v0.21.0 | Era 2 (2022–2024, 16.3M downloads) | **Different crate** — plural → singular rename (`blobs` → `blob`) | [crates.io](https://crates.io/crates/azure_storage_blobs) |
| `azure_storage` v0.21.0 | Era 2 (shared storage dependency, 16.5M downloads) | **Different crate** — replaced by `azure_storage_common` | [crates.io](https://crates.io/crates/azure_storage) |
| `azure_sdk_storage_blob` v0.45.3 | Era 1 (MindFlavor, 2017–2019, 69K downloads) | **Different crate** — community-era blob client | [crates.io](https://crates.io/crates/azure_sdk_storage_blob) |

### Migration Guide Should Cover

- Crate rename: `azure_storage_blobs` → `azure_storage_blob` (drop the plural)
- Base crate change: `azure_storage` → `azure_storage_common`
- Massive API expansion: leases, tags, SAS URLs, snapshots/versioning, page blobs, append blobs
- New `exists()` convenience method
- `filter_blobs` API
- Updated client builder pattern
- Migration path for `azure_sdk_storage_blob` users (start from scratch)
- Cargo.toml dependency changes

### Acceptance Criteria

- [ ] Migration guide published in repo docs (e.g., `docs/migration/azure_storage_blob.md`)
- [ ] Code samples for key scenarios: creating a blob client, upload/download, list blobs, leases, SAS URLs
- [ ] FAQ section covering common migration pitfalls (especially the plural → singular rename)
- [ ] Linked from the `azure_storage_blob` crate README
- [ ] Reviewed by SDK team

Start by locating the repository's existing migration guides and the `azure_storage_blob` crate README, then review the current client and API examples. Add `docs/migration/azure_storage_blob.md` with the requested scenarios, dependency changes, migration pitfalls, and FAQ; link it from the crate README and obtain SDK team review.

Written by the indexing model from the issue text.