docs: Update documentation about Sass options

[alecglassford]

Fixes #563: The doc had incorrect/outdated information about the renderSync option). The update notes that renderSync is always used as of svelte-preprocess 5.0.0.

While addressing this, I also added a more specific cross reference link for users to see which options can be passed: https://sass-lang.com/documentation/js-api/interfaces/LegacyStringOptions

I also removed the table, since that formatting now longer seemed useful, and made other minor wording adjustments to this section.

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Tests

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[kaisermann]

Hey @alecglassford 👋 thanks for catching that! 🎉