Version Warning logic - might be useful for Page Not Found

[coding-to-music]

• Add the ability to specify the version for a specific page.
• This allows you to put metadata at the top of your document:: :version: 2.3
• This version should be the version of the project that this page or API targets.
• If you configure versionwarning-node to True (default), then it will generate an in-page warning for out of date versions.
• If you configure versionwarning-console to True (default), then it will output a warning on the console.

https://github.com/readthedocs/readthedocs-sphinx-ext/blob/master/readthedocs_ext/versionwarning.py