add button to toggle dark/light mode

[xrstf]

This adds a simple button to the page to toggle between a light and dark mode. The dark mode is based on Bootstrap colors:

[tengqm]

This is nice.
Maybe we can add this to the k8s.io/docs website as well?

[tengqm]

/lgtm
/approve

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[xrstf]

For the Kubernetes website it might make sense to wait for it to use Bootstrap 5, which has better native support for it. This implementation here that I made is quite cheap, for the full website we probabl want something more robust/fine-tuned. But that's beyond my capabilities :-)

[sftim]

See kubernetes/website#37444

I'd prefer to have the button act only as an override for the user's own preference, when that preference is available.