Migration guide: `azure_security_keyvault_secrets` (from azure_security_keyvault)

Part of #4228

## Migration guide: `azure_security_keyvault_secrets`

### Official Crate

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

### 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: explain that `azure_security_keyvault` was split into `azure_security_keyvault_secrets`, `azure_security_keyvault_keys`, and `azure_security_keyvault_certificates`
- Import path changes (from unified crate to dedicated secrets crate)
- New client builder pattern
- Challenge-based authentication for sovereign clouds
- Key Vault 7.6 GA API alignment
- Updated error types and response models
- Cargo.toml dependency changes

### Acceptance Criteria

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

Start with the azure_security_keyvault_secrets crate README and the proposed docs/migration/azure_security_keyvault_secrets.md path. Document the crate split, imports, client builder, authentication, API and error changes, then add the requested secret-operation samples and FAQ; done when the guide is published, linked from the README, and ready for SDK team review.

Written by the indexing model from the issue text.