Enhanced Select: Collect HTML/SCSS documentation under its own Variant subsection

[kfranqueiro]

Logging this separately to not hold up #3608.

Consider moving the html/scss parts about Enhanced Select out of Basic Usage to within a subheading in the Variants section (it can be the first one, to make further mentions under other variants make sense).

That way we can put the html and scss right next to each other which will make them make more sense in context. As-is, you have to bounce back and forth between other content in between to realize the connection with demo-width-class.

Creating a variant subsection dedicated to enhanced select would also give us a place to discuss the usability differences/trade-offs between the two.