Add Buildkite preview links to PRs (elastic#4390)

* Create new action

* Fix preview-path for Security docs

.github/workflows/docs-preview.yml

@@ -0,0 +1,19 @@
+    name: docs-preview
+
+    on:
+      pull_request_target:
+        types: [opened]
+
+    permissions:
+      pull-requests: write
+
+    jobs:
+      doc-preview-pr:
+        runs-on: ubuntu-latest
+        steps:
+          - uses: elastic/docs/.github/actions/docs-preview@master
+            with:
+              github-token: ${{ secrets.GITHUB_TOKEN }}
+              repo: ${{ github.event.repository.name }}
+              preview-path: 'guide/en/security/master/index.html'
+              pr: ${{ github.event.pull_request.number }}