add step to verify docs on area/documentation PRs

Signed-off-by: Sebastian Malton <sebastian@malton.name>

.azure-pipelines.yml

@@ -162,3 +162,27 @@ jobs:
         condition: "and(succeeded(), startsWith(variables['Build.SourceBranch'], 'refs/tags/'))"
         env:
           NPM_TOKEN: $(NPM_TOKEN)
+---
+variables:
+  YARN_CACHE_FOLDER: $(Pipeline.Workspace)/.yarn
+pr:
+  - master
+  - releases/*
+trigger:
+  branches:
+    include:
+    - '*'
+  tags:
+    include:
+    - "area/documentation"
+jobs:
+  - job: Check-Docs
+    pool:
+      vmImage: ubuntu-16.04
+    strategy:
+      matrix:
+        node_12.x:
+          node_version: 12.x
+    steps:
+      - displayName: Verify that docs build
+        script: yarn run mkdocs-verify