Docs CI: Limit concurrent builds

.github/workflows/ci.yml

@@ -1,8 +1,13 @@
 name: Continuous integration
+
 on:
   push:
   pull_request:
 
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
 jobs:
   build:
     runs-on: ubuntu-20.04