[Documentation] Include azure devops documentation in top level command group details.

In each of the top level commands - az boards, az pipelines, az repos, az artifacts and az devops, we include the line indicating that this command is a part of azure-devops extension. In addition to this, we need to include the link out to the documentation - https://docs.microsoft.com/en-us/azure/devops/cli/?view=azure-devops as well.

https://github.com/Azure/azure-cli-extensions/issues/1454