Fix GitHub Pages deploy: add configure-pages step

[Copilot]

The deploy job was failing with a 404 because GitHub Pages was not configured to use GitHub Actions as its deployment source — actions/deploy-pages requires this to be set before it can create a deployment.

Changes

• .github/workflows/docs.yml: Added actions/configure-pages@v5 to the build job, before the mkdocs build step. This action enables GitHub Pages on the repo and sets GitHub Actions as the deployment source, which is the prerequisite for actions/deploy-pages@v4 to succeed.

steps:
  - uses: actions/checkout@v4
  - uses: actions/configure-pages@v5   # ← added
  - uses: actions/setup-python@v5
  ...