Docs: Add conceptual documentation for System.Threading.Channels

[JamesNK]

System.Threading.Channels has API docs, but there is no high-level documentation about what it is, why and when it is useful, how to use it correctly, etc.

System.Threading.Channels is a complex set of APIs and really needs conceptual docs to help developers be successful with it.

There are some quality community blog posts on channels, but nothing on docs.microsoft.com

Community blog posts:

• https://www.stevejgordon.co.uk/an-introduction-to-system-threading-channels
• https://ndportmann.com/system-threading-channels/

[MaximRouiller]

Adding customer feedback from Reddit.

https://www.reddit.com/r/dotnet/comments/i4uwu6/net_docs_whats_new_for_july_2020/g0neumg