.github: Label doc/releases PRs so tibdex/backport@v2 can backport

Signed-off-by: David Galloway <dgallowa@redhat.com>

.github/labeler.yml

@@ -1,6 +1,15 @@
 api-change:
   - src/pybind/mgr/dashboard/openapi.yaml
 
+backport quincy:
+  - doc/releases/**
+
+backport pacific:
+  - doc/releases/**
+
+backport octopus:
+  - doc/releases/**
+
 build/ops:
   - "**/CMakeLists.txt"
   - admin/**

.github/workflows/label-releases-doc-prs.yml

@@ -0,0 +1,21 @@
+---
+name: "Label doc/releases PRs so our backport GitHub Action picks them up"
+
+on:
+  pull_request:
+    types:
+      - closed
+    branches:
+      - main
+    paths:
+      - 'doc/releases/**'
+
+jobs:
+  if_merged:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Assign labels based on modified files
+        uses: actions/labeler@9794b1493b6f1fa7b006c5f8635a19c76c98be95
+        with:
+          sync-labels: ''
+          repo-token: "${{ secrets.GITHUB_TOKEN }}"