Add docs for Custom Remote Connections Feature

[sdwheeler]

Prerequisites

• Write a descriptive title.
• Search the existing issues.

Summary of the new feature or changed behavior

• Add Custom Remote Connections Feature (#17011)

These changes open up the internal PowerShell remoting API so that custom connections can be created as PowerShell modules. This PR does not introduce any new functions, and instead selectively makes some previously internal APIs public so that third parties can create custom remote connections without modifying PowerShell core engine code. See the RFC for more details.

List of articles that need to be updated

7.3 release notes

Link to related PR in PowerShell/PowerShell repo

PowerShell/PowerShell#17011

Link to related Issues in PowerShell/PowerShell repo

https://github.com/PowerShell/PowerShell-RFC/blob/master/Archive/Draft/RFC0063-Custom-Remote-Connections.md

[sdwheeler]

@michaeltlombardi See https://github.com/PowerShell/PowerShell-RFC/pull/320/files#diff-3b32295c4bdc846b2ded35b330eb91e25fd1767f46cdcb22db7d4e6dfa7b7fbcR131-R137 for items are not supported.