Document shared Clipboard/OLE architecture between WinForms and WPF (advantages and risks)

There is currently no dedicated documentation explaining:
- The mechanics of the shared Clipboard/OLE code between WinForms and WPF (e.g., how System.Private.Windows.Core and related code is packaged and used by both frameworks)
- How changes in one repo (WinForms) can directly affect WPF callers
- The advantages of this shared implementation (e.g., fixes and improvements propagate to both)
- The liabilities and risks (e.g., unintended breaking changes/regressions, broader validation requirements, ownership ambiguities)

Please create a new documentation file (under docs/ or another suitable location) that covers:
- Which clipboard/OLE code is shared, what assemblies/components are involved
- The transport/package mechanism that delivers these bits to both WinForms and WPF
- Case studies/examples (e.g., dotnet/winforms#14308, dotnet/winforms#14257, dotnet/winforms#14296)
- Change management and review recommendations
- Tradeoffs: what problems does sharing solve, what new coordination is required

The absence of such documentation increases engineering risk and slows onboarding. An explicit doc will help maintainers, community contributors, and downstream teams understand implications and minimize surprise regressions in either framework.

(Created by Copilot)