Migration guide: `azure_security_keyvault_keys` (from azure_security_keyvault)

Part of #4228

## Migration guide: `azure_security_keyvault_keys`

### Official Crate

- **Name:** [`azure_security_keyvault_keys`](https://crates.io/crates/azure_security_keyvault_keys)
- **Current version:** 0.13.0
- **Service:** Azure Key Vault Keys

### Unofficial Predecessor(s)

| Predecessor | Version(s) | Relationship | crates.io |
|---|---|---|---|
| `azure_security_keyvault` v0.1.0–0.21.0 | Era 2 (2022–2024) | **Different crate** — era 2 monolith covering secrets, keys, and certificates; split into 3 dedicated crates in era 3 | [crates.io](https://crates.io/crates/azure_security_keyvault) |

### Migration Guide Should Cover

- The Key Vault crate split (same context as secrets guide)
- Dedicated key operations API surface
- Key Vault 7.6 GA support
- Key attestation feature
- New cryptographic algorithms
- Challenge-based authentication for sovereign clouds
- Complete API surface change from unified crate
- Cargo.toml dependency changes

### Acceptance Criteria

- [ ] Migration guide published in repo docs (e.g., `docs/migration/azure_security_keyvault_keys.md`)
- [ ] Code samples for key scenarios: creating a keys client, create/get/delete/list keys, key rotation, crypto operations
- [ ] FAQ section covering common migration pitfalls (especially the crate split)
- [ ] Linked from the `azure_security_keyvault_keys` crate README
- [ ] Reviewed by SDK team

Begin with the azure_security_keyvault_keys crate and its README, then review the predecessor azure_security_keyvault context described in the issue. Create docs/migration/azure_security_keyvault_keys.md with the requested API examples, migration pitfalls, authentication and feature coverage, and link it from the crate README. Done means all acceptance criteria are addressed and the SDK team has reviewed the guide.

Written by the indexing model from the issue text.