docs: document asynchronous networking

[aazainjan]

Summary

• Documented Socketwrapper's asynchronous networking functionality
• Explained callback-based operations
• Documented future-based operations
• Added an overview of C++20 coroutine support
• Documented async_run(), block_on(), and spawn()

Motivation

The README previously contained a TODO for the asynchronous functionality.
This change replaces the TODO with documentation for the asynchronous APIs
already provided by the library.

[tglane]

Hi and thank you for your PR. You've added documentation for the callback based asynchronous functions but not for the ones that return a std::promise or an awaitable that's useable in coroutines. Could you add that, too?