Decouple `sx` prop from system and add docs

[siriwatknp]

I decided to change the intention of this PR so that we can ship incrementally.

From Pigment CSS users, sx prop gives a convenient way to style elements without writing any import. The shorthand should not be enabled by default (it's too magical). If they want shorthand, they need to add some theme config (a new feature for Pigment CSS).

So this PR is about decoupling the shorthand from System so that sx prop does not come with shorthand by default.

Then consider shorthand as another feature that can be worked separately.

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