Move `postcss-css-variables` out of `Future CSS Syntax` section
Move `postcss-css-variables` out of `Future CSS Syntax` section

Since this plugin does not provide a future proof code (see https://github.com/MadLittleMods/postcss-css-variables/issues/4) and documentation is still misleading and unclear about the differences, I think it does not belong into this section.

[skip ci]

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