[KeyVault] - Review the contributing guide and add information about secure key release

[maorleger]

Secure Key Release requires an additional test resource (an attestation service) that we deploy from a container managed by EngSys.

To make contributing simpler, add information about the dependencies needed for live testing, where the source code is located, and how to get started with managed HSM SKR.

In addition, let's revisit https://github.com/Azure/azure-sdk-tools/tree/main/tools/keyvault-mock-attestation and add better documentation, clearer instructions for docker, etc.