docs: add a lifetime and ownership page for composing coroutines

[sgerbino]

From the review result:

Capy: add a lifetime and ownership section covering the issues that users need to understand when composing Capy coroutines.

Source: Boost review result (conditional acceptance), Boost mailing list, September 2026.

Lifetime guidance is currently scattered: lambda captures (4h), continuation dangling (4d), destroy() on suspended handles (4e), when_all captures (4f), buffer reference params (5a). No single page gathers the rules for arguments by reference, frame lifetimes, and shutdown ordering.