Documentation: Adding Classic Azure DevOps details (#471)

There is a lack of documentation about how to set up PR Metrics using classic (UI) pipelines in Azure DevOps, which can cause confusion, particularly around gaining access to secrets. This change adds the relevant information, making it easier for people to set this up.

docs/azure-pipelines-task.md

@@ -66,6 +66,29 @@ steps:
   continueOnError: true
 ```
 
+## Classic Pipelines
+
+For classic pipelines, you need to specify the secret as an environment
+variable.
+
+First, create a new environment variable in the pipeline definition under the
+variables tab with the name `PR_Metrics_Access_Token` and the value of the PAT
+as outlined above. Mark the variable as a secret to ensure it is not exposed in
+the logs.
+
+![Screenshot of the variable definition](images/variable-definition.png)
+
+Then, add the task to the pipeline and expose the secret as an environment
+variable. To do this, expand the "Environment Variables" section and press "+"
+to add a new variable. In the first text box, add `PR_METRICS_ACCESS_TOKEN` and
+in the second, add `$(PR_Metrics_Access_Token)`.
+
+![Screenshot of the task definition](images/task-definition.png)
+
+You can use a name other than `PR_Metrics_Access_Token` if you wish, but you
+will need to keep the name unified across the variable and task definitions. The
+name `PR_METRICS_ACCESS_TOKEN` cannot be altered.
+
 ## Always Close Comment
 
 The `AlwaysCloseComment` option is not available for GitHub PRs as the main size