Docs: Use sphinx-notfound-page to show a nicer 404 page

[hugovk]

Currently we have the default nginx 404:

Use the https://pypi.org/project/sphinx-notfound-page/ extension to show a nicer 404 page:

We're also using this at https://devguide.python.org/

I kept the defaults here but we can configure the title and text, or template:

• https://sphinx-notfound-page.readthedocs.io/en/latest/configuration.html

Let's not backport this immediately; I'd like to check it at https://docs.python.org/dev/ first.

---

📚 Documentation preview 📚: https://cpython-previews--111084.org.readthedocs.build/

[hugovk]

Let's try this on https://docs.python.org/dev/ first.