ci: Cancel all workflows consistently in pull requests without blocking main

The previous setup would cause multiple commits on main to be stuck in a pending
state, waiting for the previous run to be finished.

The new group specification is taken from:
https://docs.github.com/en/actions/using-jobs/using-concurrency#example-using-a-fallback-value

This also adds the same settings for the docs workflow.

.github/workflows/ci.yaml

@@ -13,9 +13,9 @@ on:
       - v[0-9]+
 
 concurrency:
-  group: ${{ github.workflow }}-${{ github.ref }}
   # Terminate all previous runs of the same workflow for pull requests
-  cancel-in-progress: "${{ github.event_name == 'pull_request' }}"
+  group: ci-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
 
 jobs:
   Lint-Style:

.github/workflows/docs.yaml

@@ -10,6 +10,11 @@ on:
     - main
     - v[0-9]+
 
+concurrency:
+  # Terminate all previous runs of the same workflow for pull requests
+  group: docs-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
 jobs:
   build:
     name: Build docs