asyncio: Pass cancellation from wrapping Future to wrapped Future. By Saúl Ibarra Corretgé (mostly).

Closes #11710: create "landing pages" (/library/package.html) for those packages that have no documented content themselves, e.g. "urllib" or "http".

Break up the 'someos' docs classification based on a more user-focused scheme, including creation of a separate 'Concurrent Execution' section