Fix GitHub pages workflow bugs (#1128)

      with:

        persist-credentials: false

    - uses: actions-rs/toolchain@v1

      with:

        toolchain: stable

    - name: Generate docs

      env:

        GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

        mdbook build --dest-dir ../../output

        popd &>/dev/null

        git checkout origin/github-pages

        git checkout origin/github-pages -b github-pages

        rm -rf design

        mv ../output design

        git add design