docs: link the related library family (Corosio, Http, Websocket, Beast2, Burl)

[sgerbino]

From the Corosio Boost review feedback:

A quick review of the documentation reveals a number of such libraries,
planned or otherwise:

• Corosio — portable coroutine networking (this review)
• Http — sans-I/O HTTP/1.1 clients and servers
• Websocket — sans-I/O WebSocket
• Beast2 — high-level HTTP/WebSocket servers
• Burl — high-level HTTP client

Note to the authors - I'd like to see links to these libraries in the
docs - even if that directs to a "coming soon" page or repo under
development. For our needs we'd definitely be looking to utilise
WebSocket functionality that allows us to replace (or integrate with) [...]

The documentation should link to the related libraries in the family so readers can see where each layer lives, even where a library is still under development:

• Corosio - portable coroutine networking
• Http - HTTP/1 parsing and serialization (sans-I/O)
• Websocket - low-level WebSocket protocol (sans-I/O)
• Beast2 - high-level HTTP/1 and WebSocket
• Burl - HTTP client for C++20

Tasks:

• Add a section to the docs introducing the library family and linking each repository
• Mark libraries that are under development as such