add Azure Pipelines Scaler doc (#411)

Co-authored-by: Tom Kerkhove <kerkhove.tom@gmail.com>

content/docs/2.3/scalers/azure-pipelines.md

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+title = "Azure Pipelines"
+layout = "scaler"
+availability = "v2.3+"
+maintainer = "Community"
+description = "Scale applications based on agent pool queues for Azure Pipelines."
+go_file = "azure_pipelines_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `azure-pipelines` trigger for Azure Pipelines. It scales based on the amount of pipeline runs pending in a given agent pool.
+
+```yaml
+triggers:
+  - type: azure-pipelines
+    metadata:
+      # Required: poolID - Can be retreived by the REST API call https://dev.azure.com/{organizationName}/_apis/distributedtask/pools?poolname={agentPoolName}
+      poolID: "1"
+      # Optional: Azure DevOps organization URL, can use TriggerAuthentication as well
+      organizationURLFromEnv: "AZP_URL"
+      # Optional: Azure DevOps Personal Access Token, can use TriggerAuthentication as well
+      personalAccessTokenFromEnv: "AZP_TOKEN"
+      # Optional: Target queue length
+      targetPipelinesQueueLength: "1" # Default 1
+    authenticationRef:
+     name: pipeline-trigger-auth
+```
+
+**Parameter list:**
+
+- `poolID` - Id of the queue.
+- `organizationURLFromEnv` - Name of the environment variable your deployment uses to get the URL for your Azure DevOps organization.
+- `personalAccessTokenFromEnv` - Name of the environment variable that provides the personal access token (PAT) for Azure DevOps. Learn more about how to create one [in the official docs](https://docs.microsoft.com/en-us/azure/devops/organizations/accounts/use-personal-access-tokens-to-authenticate?view=azure-devops&tabs=preview-page).
+- `targetPipelinesQueueLength` - Target value for the amount of pending jobs in the queue to scale on. (default: 1)
+  - Example - If one pod can handle 10 jobs, set the queue length target to 10. If the actual number of jobs in the queue is 30, the scaler scales to 3 pods.
+
+### Authentication Parameters
+
+As an alternative to using environment variables, you can authenticate with Azure Devops using a Personal Access Token via `TriggerAuthentication` configuration.
+
+**Personal Access Token Authentication:**
+
+- `organizationURL` - The URL of the Azure DevOps organization
+- `personalAccessToken` - The Personal Access Token (PAT) for Azure DevOps
+
+### Example
+
+```yaml
+apiVersion: v1
+kind: Secret
+type: Opaque
+metadata:
+  name: pipeline-auth
+data:
+  personalAccessToken: <encoded personalAccessToken>
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: pipeline-trigger-auth
+  namespace: default
+spec:
+  secretTargetRef:
+    - parameter: personalAccessToken
+      name: pipeline-auth
+      key: personalAccessToken
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: azure-pipelines-scaledobject
+  namespace: default
+spec:
+  scaleTargetRef:
+    name: azdevops-deployment
+  minReplicaCount: 1
+  maxReplicaCount: 5 
+  triggers:
+  - type: azure-pipelines
+    metadata:
+      poolID: "1"
+      organizationURLFromEnv: "AZP_URL"
+    authenticationRef:
+     name: pipeline-trigger-auth
+```