ci: docs pr checker (#845)

## fixes KILTprotocol/ticket#2890
Make docs PR mandatory


## Checklist:

- [ ] I have verified that the code works
- [ ] No panics! (checked arithmetic ops, no indexing `array[3]` use
`get(3)`, ...)
- [ ] I have verified that the code is easy to understand
  - [ ] If not, I have left a well-balanced amount of inline comments
- [ ] I have [left the code in a better
state](https://deviq.com/principles/boy-scout-rule)
- [ ] I have documented the changes (where applicable)
* Either PR or Ticket to update [the
Docs](https://github.com/KILTprotocol/docs)
    * Link the PR/Ticket here

Author: ggera

.github/workflows/check-docs-pr.yml

@@ -0,0 +1,43 @@
+name: Docs PR Check
+
+on:
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+jobs:
+  check-docs-pr:
+    runs-on: ubuntu-latest
+    env:
+      GH_TOKEN: ${{ secrets.GH_TOKEN }}
+
+    steps:
+      - name: Checkout Code
+        uses: actions/checkout@v4
+
+      - name: Check for Skip Label
+        run: |
+          if gh pr view ${{ github.event.pull_request.number }} --json labels --jq '.labels | .[].name' | grep -q 'ci-skip-docs-pr'; then
+            echo "SKIP_DOCS_PR_CHECK=true" >> $GITHUB_ENV
+          else
+            echo "SKIP_DOCS_PR_CHECK=false" >> $GITHUB_ENV
+          fi
+
+      - name: Ensure Linked Docs PR
+        if: env.SKIP_DOCS_PR_CHECK == 'false'
+        env:
+          DOCS_REPO: KILTprotocol/docs
+          PR_NO: ${{ github.event.pull_request.number }}
+        run: |
+          DOCS_PRS=$(gh pr list --repo $DOCS_REPO --json number,body,title --jq '.[]')
+
+          MATCHING_PR=$(echo "$DOCS_PRS" | jq -r "select(.body | contains(\"#${PR_NO}\") or .title | contains(\"#${PR_NO}\"))")
+
+          if [ -z "$MATCHING_PR" ]; then
+            echo "Please link a docs PR for code PR #${PR_NO} or apply the 'ci-skip-docs-pr' label."
+            exit 1
+          else
+            echo "Matching docs PR found for code PR #${PR_NOPR_NO}."
+          fi