Fixed spell checking in docs GitHub Actions workflow.

The spelling check job was passing even with spelling errors because
the system spell checker (`aspell`) was not installed on the GitHub
Actions runner. While `sphinxcontrib.spelling` and `PyEnchant` were
installed via pip, they require a system-level spell checker backend
to function.

.github/workflows/docs.yml

@@ -35,6 +35,8 @@ jobs:
           python-version: '3.14'
           cache: 'pip'
           cache-dependency-path: 'docs/requirements.txt'
+      - name: Install system spell checker
+        run: sudo apt update && sudo apt install -y aspell aspell-en
       - run: python -m pip install -r docs/requirements.txt
       - name: Build docs
         run: |