Add WebSockets conceptual doc

[CarnaViire]

Currently we don't have any conceptual docs regarding WebSockets at all. We should add them to the Networking area, that already contains HTTP and Sockets docs: https://github.com/dotnet/docs/tree/main/docs/fundamentals/networking

Things to cover:

• Brief explanation of protocol and examples
• Compression (permessage-deflate extension) and its dangers
• HTTP/2 WebSockets
  • Difference between H/2 and H/1.1 WebSockets and what's the gain
  • How to use H/2 and how to enable automatic upgrade/downgrade of HTTP version
• Passing a custom HttpMessageInvoker/HttpClient and incompatible options (related to [Breaking change]: ClientWebSocket.ConnectAsync throws ArgumentException on incompatible options #31430)