Update Sass and Theme docs to document mixins and functions better

[mdo]

Also remove some unused functions while we're here.

[pricop]

Personally, I'm making use of the negativify-map in when generating utilities such as: such as rotate, top, right, bottom, left, vertical-align.

Or perhaps there's an easier way to achieve this that I'm missing?

I'm personally using Tailwind's $spacers which works in .25rem units and it's a breeze to position elements wherever I need them. For example I'm using negative top top-n4.

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Another example where I'm using negative top right-n4 and bottom-n4:

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Another example where I'm using rotate-n45:

Also, for rotate utility, we need both directions because sometimes you may want to animate a "wiggle" effect, and this wouldn't be possible otherwise.

Unless I'm missing something, I'd say you should keep this function as a helper function...

[mdo]

I didn't necessarily want to generate a negative value for every existing key though, so TBD on that. I think leaving the option to others makes sense but open to hearing more feedback.

[pricop]

Totally understandable, just having the option to do it would be good enough.