[Enhancement]: Auto option for light/dark toggle

### What you'd like to propose?

Currently, the light/dark theme toggle has three states; the first is a hidden "auto" state which can only be accessed by never clicking the toggle, or by deleting all your history (see #1382). I think it would be much nicer to have this state show up in the toggle, so it would be 🌓 -> 🌞 -> 🌙. I do that in https://halogenica.net/beautifulhugo for a different theme. Personally, I nearly always want the system settings to be respected; if I tell my system to be in dark mode, I don't want random pages to flash-bang me at night. But I've clicked that little icon when testing my site (https://scikit-build.org, if you are curious), and I can't get the auto mode back now. I could remove the toggle in the config, but then some people do seem to like per-page settings (not sure why, except when I'm testing light/dark mode :) ), and I'm also not sure if that would be a problem if they've already changed this off auto by clicking the button.

### Code of Conduct

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