Docs: Highlight the IoAwaitable dimension of the library

[akrzemi1]

This message from the Boost Review surfced another motivation for Capy, currently not reflected in the documentation:
https://lists.boost.org/archives/list/boost@lists.boost.org/thread/XTGEPSMUUAVYFSHXH242S7KB66UINLNX/

Change the docs so that it is clear from the first page that there is a smaller "library" inside Capy aiming to be a vocabulary type for communicating between any awaitable-based coroutine libraries.