DPE-2806 discourse gatekeeper (#85)

Port of k8s
[PR#306](canonical/mysql-k8s-operator#306)

Author: paulomach

.github/workflows/sync_docs.yaml

@@ -0,0 +1,33 @@
+name: Sync docs from Discourse
+
+on:
+  workflow_dispatch:
+  schedule:
+    - cron: '53 0 * * *' # Daily at 00:53 UTC
+  push:
+    branches:
+      - main
+
+jobs:
+  sync-docs:
+    name: Open PR with docs changes
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write  # Needed to login to Discourse
+      pull-requests: write # Need to create PR
+    steps:
+      - uses: actions/checkout@v3
+      - name: Open PR with docs changes
+        uses: deusebio/discourse-gatekeeper@c8adb89ea1cbceca54d78da798658373615487ac
+        id: docs-pr
+        with:
+          discourse_host: discourse.charmhub.io
+          discourse_api_username: ${{ secrets.DISCOURSE_API_USERNAME }}
+          discourse_api_key: ${{ secrets.DISCOURSE_API_KEY }}
+          github_token: ${{ secrets.GITHUB_TOKEN }}
+          dry_run: "true"
+
+      - name: Show migrate output
+        run: echo '${{ steps.docs-pr.outputs.migrate }}'
+      - name: Show reconcile output
+        run: echo '${{ steps.docs-pr.outputs.reconcile }}'