Add documentation for KeyVault provider

[maddieclayton]

Description

• Identify and target KeyVault PowerShell conceptual documentation for refactoring using the new paradigm
• Add provider documentation
  • GetMamlHelp
  • Online provider docs

[maddieclayton]

Provider code here: https://github.com/Azure/azure-powershell/tree/KVprov1

[teresaritorto]

This issue has been marked as stale and is being closed.