Update docs CI

Test in CI that the docs can be built with the oldest supported
python version

.github/workflows/doc-build.yml

@@ -1,4 +1,6 @@
-# The docs should build without warnings from sphinx
+# The docs should build without warnings from sphinx.
+# It should be possible to build the docs with python3.8 so contributors
+# can work with the oldest supported python version.
 
 name: Docs build
 
@@ -14,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python: ["3.11"]
+        python: ["3.8", "3.11"]
 
     steps:
       - uses: actions/checkout@v4