x/website: Improved table styling

[deitrix]

Looking at https://go.dev/ref/mod#environment-variables, it's not immediately clear which descriptions correspond to which environment variables.

Take the following for instance, it's unclear where the description for GOPRIVATE ends and begins for GOPROXY. There are no visual cues:

What I propose is a more accessible set of styles for tables like this, with alternative coloring and padding around td and th elements. This should help distinguish between rows:

Special attention should be paid to the selection of the alternating row colors, and how they interact with the coloring of things like code blocks.

[gabyhelp]

Related Issues

• x/website: tables are awkwardly formatted #67368

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