Use new split workflow Ubuntu 24.04 in workflows (frequenz-floss#369)

These new workflows also start using a new reusable `gh-action-nox`,
native arm runners and Ubuntu 24.04, as [Ubuntu 20.04 will be removed
from GitHub runners by April's
1st](https://github.blog/changelog/2025-01-15-github-actions-ubuntu-20-runner-image-brownout-dates-and-other-breaking-changes/
). Python 3.12 is also added to the test matrix.

The PR action is more lightweight, and only tests with one matrix (the
most widely used), so PRs can be tested more quickly.

The push workflow does a more intense testing with all matrix
combinations. This also runs for the merge queue, so before PRs are
actually merged the tests will run for the complete matrix.

Fixes frequenz-floss#342, frequenz-floss#341. Improves frequenz-floss#59.

Author: llucax

.github/workflows/ci-pr.yaml

@@ -0,0 +1,55 @@
+name: Test PR
+
+on:
+  pull_request:
+
+env:
+  # Please make sure this version is included in the `matrix`, as the
+  # `matrix` section can't use `env`, so it must be entered manually
+  DEFAULT_PYTHON_VERSION: '3.11'
+  # It would be nice to be able to also define a DEFAULT_UBUNTU_VERSION
+  # but sadly `env` can't be used either in `runs-on`.
+
+jobs:
+  nox:
+    name: Test with nox
+    runs-on: ubuntu-24.04
+
+    steps:
+      - name: Run nox
+        uses: frequenz-floss/[email protected]
+        with:
+          python-version: "3.11"
+          nox-session: ci_checks_max
+
+  test-docs:
+    name: Test documentation website generation
+    runs-on: ubuntu-24.04
+    steps:
+      - name: Setup Git
+        uses: frequenz-floss/[email protected]
+
+      - name: Fetch sources
+        uses: actions/checkout@v4
+        with:
+          submodules: true
+
+      - name: Setup Python
+        uses: frequenz-floss/[email protected]
+        with:
+          python-version: ${{ env.DEFAULT_PYTHON_VERSION }}
+          dependencies: .[dev-mkdocs]
+
+      - name: Generate the documentation
+        env:
+          MIKE_VERSION: gh-${{ github.job }}
+        run: |
+          mike deploy $MIKE_VERSION
+          mike set-default $MIKE_VERSION
+
+      - name: Upload site
+        uses: actions/upload-artifact@v4
+        with:
+          name: docs-site
+          path: site/
+          if-no-files-found: error