WIP: docs(guide/accessibility): explain ngAria

Work-in-progress.

ping @marcysutton
`aria-hidden` is actually unnecessary here, since an element hidden with either `ngShow` or `ngHide` already has `display: none !important;` on it (at least in my demo, it did). That pulls the item out of the accessibility tree, which is what `aria-hidden="true"` does. Are there ever cases where `ngShow`/`ngHide` simply changes visibility with `opacity` or similar, keeping it in the tree? That's where `aria-hidden` would be needed.

We might want to highlight a different property, such as `aria-required`, which communicates the nature of a custom control to assistive technology.  Such as:
```html
<material-checkbox required>
```
Which becomes:
```html
<material-checkbox required aria-required="true">
```
ARIA states & properties like these are useful when an attribute isn't valid on a custom element directive or a `<div>`. Valid, meaning it would communicate the nature of the element via accessibility APIs. Otherwise, they are redundant.