ci: label PRs with changelog needed (#2546)

.github/labeler_merged.yml

+needs changelog:
+  - all: ['!docs/changelog.rst']

.github/workflows/pr_merged.yml

+name: PR merged
+on:
+  pull_request:
+    - closed
+
+jobs:
+  label-merged:
+    name: Changelog needed
+    runs-on: ubuntu-latest
+    if: github.event.pull_request.merged == true
+    steps:
+    - uses: actions/labeler@main
+      with:
+        repo-token: ${{ secrets.GITHUB_TOKEN }}
+        configuration-path: .github/labeler_merged.yml