Add intro and context to ARIA Notify

As noted by @matatk in https://github.com/w3c/a11y-request/issues/165, we should have some sort of intro and context for ARIA Notify. I am happy to take this one on by reusing the [explainer](https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Accessibility/AriaNotify/explainer.md)