Use ARIA attributes for states

[simurai]

This PR:

• Adds support for [aria-selected=true] Applies .selected styles to [aria-selected=true] #982
• Adds support for [aria-current=page]
• Adds support to use <button> elements for the UnderlineNav f63d53c
• Updates the docs to only use aria attributes instead of the .selected and .current classes.
• A few other docs improvements and clean up.

A typical change looks like this:

&.selected,
+ &[aria-selected=true]
{...}

- <a class="UnderlineNav-item selected">
+ <a class="UnderlineNav-item" aria-selected="true">

So it's still possible to use the .selected class for now, but not encouraged anymore.

And by only using [aria-current] as selector, it allows the value to also be true, like in aria-current="true" in case it's not a "page" and used for changing tabs without navigating to a new URL.

---

Closes #790

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[jonrohan]

Feels good to support more aria-selected and aria-current 👍