[AriaNotify] Move to a community or working group

I'm in a session of the ARIA WG that's discussing https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/Accessibility/AriaNotify/explainer.md#use-cases. There's easily enough interest to move the explainer to the WICG, but there might be another more appropriate CG that feeds into ARIA, or it could be that ARIA should adopt this directly.