Github action and backportrc config files (#360)

* Github action and backportrc config files

Author: ianf-mongodb

.backportrc.json

@@ -0,0 +1,17 @@
+// .backportrc.json
+{
+    // example repo info
+    "repoOwner": "mongodb",
+    "repoName": "docs-cluster-to-cluster-sync",
+  
+    // `targetBranch` option allows to automatically backport every PR to a specific branch without the need for labels
+    "targetBranches": ["backport-gha-test"],
+  
+    // the branches available to backport to
+    "targetBranchChoices": ["backport-gha-test-2"], // will be "beta"
+  
+    // In this case, adding the label "backport-to-production" will backport the PR to the "production" branch
+    "branchLabelMapping": {
+      "^backport-to-(.+)$": "$1"
+    }
+  }

.github/workflows/backport.yml

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+name: Automatic backport action
+
+on:
+  pull_request_target:
+    types: ["labeled", "closed"]
+
+jobs:
+  backport:
+    name: Backport PR
+    runs-on: ubuntu-latest
+    steps:
+      - name: Backport Action
+        uses: sorenlouv/[email protected]
+        with:
+          github_token: ${{ secrets.BACKPORT_TOKEN }}
+          auto_backport_label_prefix: backport-to-
+
+      - name: Info log
+        if: ${{ success() }}
+        run: cat ~/.backport/backport.info.log
+        
+      - name: Debug log
+        if: ${{ failure() }}
+        run: cat ~/.backport/backport.debug.log        
+