Add workflow to publish on GH pages

[acocheo]

Workflow will:

• install npm
• run build
• run a job like this:

  # Deployment job
  deploy:
  environment:
    name: github-pages
    url: ${{ steps.deployment.outputs.page_url }}
  runs-on: ubuntu-latest
  needs: build
  steps:
    - name: Deploy to GitHub Pages
      id: deployment
      uses: actions/deploy-pages@v2