[CHORE]: Add CI workflow to make sure changelog gets updated with every PR

.github/workflows/check-changelog.yml

@@ -0,0 +1,32 @@
+name: Check changelog
+
+on:
+  pull_request:
+    types: [opened, reopened, synchronize, labeled, unlabeled, ready_for_review]
+
+concurrency:
+  group: check-changelog-${{ github.event.pull_request.number }}
+  cancel-in-progress: true  
+
+permissions:
+  contents: read
+  pull-requests: read
+
+jobs:
+  check-changelog:
+    name: Check if CHANGELOG.md was updated
+    if: ${{ !github.event.pull_request.draft }}
+    runs-on: ubuntu-latest
+    env:
+      GH_TOKEN: ${{ github.token }}
+      HAS_SKIP_LABEL: ${{ contains(github.event.pull_request.labels.*.name, 'no-changelog') }}
+    steps:
+      - run: |
+          if [ "$HAS_SKIP_LABEL" = true ]; then
+            echo "::notice::This PR has the 'no-changelog' label, so a change to CHANGELOG.md is not required."
+          elif gh api "repos/${{ github.repository }}/pulls/${{ github.event.number }}/files" --paginate --jq '.[].filename' | grep -qx CHANGELOG.md; then
+            echo "::notice::CHANGELOG.md was updated."
+          else
+            echo "::error::CHANGELOG.md was not updated. Either add a changelog entry, or add the 'no-changelog' label to this PR if a changelog entry is not needed."
+            exit 1
+          fi

CONTRIBUTING.md

@@ -181,7 +181,8 @@ When creating your pull request, please follow the guidelines below.
 - If your PR modifies code of `plotly.graph_objects`, the modifications should be made to the code generator, *not* the generated files.
 - You have added tests or modified existing tests, as needed.
 - For a new feature, you have added documentation examples (please see the doc checklist as well).
-- You have added a CHANGELOG entry if changing anything substantial.
+- You have added a changelog entry to `CHANGELOG.md` if changing anything substantial.
+  - The CI job "Check changelog" will fail if the PR does not update `CHANGELOG.md`. To bypass this check for PRs which don't require a changelog entry (e.g. docs updates), add the `no-changelog` label to the PR.
 - For a new feature or a change in behavior, you have updated the relevant docstrings in the code.
 
 ### Documentation pull request