CI: group all required workflows into one for the status check.

This simplifies repository management.

.github/workflows/main.yaml

@@ -87,6 +87,15 @@ jobs:
           name: docs
           path: docs/_build
 
+  required: # group all required workflows into one for the required status check
+    needs:
+      - test
+      - document
+    runs-on: ubuntu-latest
+    steps:
+      - run: |
+          true
+
   publish-docs:
     needs: document
     if: github.repository == 'amaranth-lang/amaranth'