add built documentation ZIP file artifact upload to CI workflow

[prjemian]

As demonstrated elsewhere, upload a ZIP-file copy of the just-built documentation with each run of the CI workflow by adding these two steps.

This step should be added early in the workflow:

    - name: Set env vars
      run: |
        export REPOSITORY_NAME=${GITHUB_REPOSITORY#*/}  # just the repo, as opposed to org/repo
        echo "REPOSITORY_NAME=${REPOSITORY_NAME}" >> $GITHUB_ENV

This step should be added just before the deploy step in the workflow:

    - name: Upload Docs ZIP file as artifact
      uses: actions/upload-artifact@v3
      with:
        name: ${{ env.REPOSITORY_NAME }}-docs
        path: docs/build/html/