Add GH action for posting docs preview (#728)

* Add GH action for posting docs preview

* Update .github/workflows/docs-preview.yml

Co-authored-by: Nassim Kammah <nkammah@gmail.com>

---------

Co-authored-by: Nassim Kammah <nkammah@gmail.com>

Author: kilfoyle

.github/workflows/docs-preview.yml

@@ -0,0 +1,20 @@
+---
+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/fleet/index.html'
+          pr: ${{ github.event.pull_request.number }}