Reintroduce desktop file

[frenzymadness]

The desktop file was inadvertently removed in: 5b6ed99

Fixes: #6704

I did check that the files are present in the sdist and wheel archives:

$ tar -tf dist/notebook-7.0.0a11.tar.gz| grep -E "notebook\.(svg|desktop)"
notebook-7.0.0a11/jupyter-notebook.desktop
notebook-7.0.0a11/notebook.svg
notebook-7.0.0a11/node_modules/@jupyterlab/ui-components/style/icons/filetype/notebook.svg

$ zip -sf dist/notebook-7.0.0a11-py3-none-any.whl | grep -E "notebook\.(svg|desktop)"
  notebook-7.0.0a11.data/data/share/applications/jupyter-notebook.desktop
  notebook-7.0.0a11.data/data/share/icons/hicolor/scalable/apps/notebook.svg

[github-actions]

👈 Launch a Binder on branch frenzymadness/notebook/desktop_file

[jtpio]

Thanks!