Add `sphinx-notfound-page` to configure styled 404 pages

[jbampton]

Popular extension:

https://sphinx-extensions.readthedocs.io/en/latest/

https://github.com/readthedocs/sphinx-notfound-page

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

See the current 404 page:

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📚 Documentation preview 📚: https://cloudstack-documentation--586.org.readthedocs.build/en/586/

[jbampton]