Cast lightness palette to a 4% grid

[aaazzam]

Our design system samples lightness values more or less randomly clustered around increments of 4%.

This PR casts our lightness values to a 4% grid.

    --p-color-bg-0: hsl(var(--p-primary-hue), 6%, 15.88%);
    --p-color-bg-1: hsl(var(--p-primary-hue), 6%, 11.76%);
    --p-color-bg-2: var(--p-color-bg-0);
    --p-color-bg-3: hsl(var(--p-primary-hue), 6%, 20.2%);
    --p-color-bg-floating: hsl(var(--p-primary-hue), 6%, 22.16%);
    --p-color-bg-floating-sticky: hsl(var(--p-primary-hue) 6.67% 11.76% / 0.9);
    --p-color-bg-code: hsl(var(--p-primary-hue), 12.2%, 8.04%);
    --p-color-bg-overlay: hsl(0.0 0.0% 0.0% / 0.5);

becomes

    --p-color-bg-0: hsl(var(--p-primary-hue), 6%, 16%);
    --p-color-bg-1: hsl(var(--p-primary-hue), 6%, 12%);
    --p-color-bg-2: hsl(var(--p-primary-hue), 6%, 16%);
    --p-color-bg-3: hsl(var(--p-primary-hue), 6%, 20%);
    --p-color-bg-floating: hsl(var(--p-primary-hue), 6%, 24%);
    --p-color-bg-floating-sticky: hsl(var(--p-primary-hue) 6% 12% / 0.9);
    --p-color-bg-code: hsl(var(--p-primary-hue), 6%, 8%);
    --p-color-bg-overlay: hsl(0.0 0.0% 0.0% / 0.5);

The color changes here are more-or-less imperceptible, and shouldn't have noticeable changes to users. This is more of a form-over-function PR.

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