[Explainer] Update to fetch-based design

- Previous discussions about fetch-based API are in WICG#70, WICG#52, WICG#50, WebKit/standards-positions#85 (comment)
- Related new discussions in WICG#71, WICG#72, WICG#73, WICG#74, WICG#75

Author: mingyc