index.html: Move to larger video to test if frameworks can handle it.

[averms]

The video is 1920 x 1080 which means that centered CSS themes will need a special rule to resize the video to fit inside the margin. From early testing only a few of the CSS dropins correctly handle this, awsm.css being one of them.

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Here is what it looks like: https://avrma-forks.github.io/dropin-minimal-css/