Fixes #4425 - ApplicationImpl internal (#4426)

* Pulled from v2_release

* Refactor migration guide for Terminal.Gui v2

Restructured and expanded the migration guide to provide a comprehensive resource for transitioning from Terminal.Gui v1 to v2. Key updates include:

- Added a Table of Contents for easier navigation.
- Summarized major architectural changes in v2, including the instance-based application model, IRunnable architecture, and 24-bit TrueColor support.
- Updated examples to reflect new patterns, such as initializers replacing constructors and explicit disposal using `IDisposable`.
- Documented changes to the layout system, including the removal of `Absolute`/`Computed` styles and the introduction of `Viewport`.
- Standardized event patterns to use `object sender, EventArgs args`.
- Detailed updates to the Keyboard, Mouse, and Navigation APIs, including configurable key bindings and viewport-relative mouse coordinates.
- Replaced legacy components like `ScrollView` and `ContextMenu` with built-in scrolling and `PopoverMenu`.
- Clarified disposal rules and introduced best practices for resource management.
- Provided a complete migration example and a summary of breaking changes.

This update aims to simplify the migration process by addressing breaking changes, introducing new features, and aligning with modern .NET conventions.

* Refactor to use Application.Instance for lifecycle management

Replaced all occurrences of `ApplicationImpl.Instance` with the new `Application.Instance` property across the codebase to align with the updated application lifecycle model.

Encapsulated the `ApplicationImpl` class by making it `internal`, ensuring it is no longer directly accessible outside its assembly. Introduced the `[Obsolete]` `Application.Instance` property as a backward-compatible singleton for the legacy static `Application` model, while encouraging the use of `Application.Create()` for new code.

Updated `MessageBox` methods to use `Application.Instance` for consistent modal dialog management. Improved documentation to reflect these changes and emphasize the transition to the instance-based application model.

Performed code cleanup in multiple classes to ensure consistency and maintainability. These changes maintain backward compatibility while preparing the codebase for the eventual removal of the legacy `ApplicationImpl` class.

* Fix doc bug

* - Removed obsolete `.cd` class diagram files.
- Introduced `IRunnable` interface for decoupling component execution.
- Added fluent API for running dialogs and retrieving results.
- Enhanced `View` with `App` and `Driver` properties for better decoupling.
- Improved testability with support for mock and real applications.
- Implemented `IDisposable` for proper resource cleanup.
- Replaced `RunnableSessionStack` with `SessionStack` for session management.
- Updated driver architecture to align with the new model.
- Scoped `IKeyboard` to application contexts for modularity.
- Updated documentation with migration strategies and best practices.

These changes modernize the library, improve maintainability, and align with current development practices.

docfx/docs/keyboard.md

@@ -230,7 +230,7 @@ The @Terminal.Gui.App.Keyboard class implements @Terminal.Gui.App.IKeyboard and
 - **Events**: KeyDown, KeyUp events for application-level keyboard monitoring
 - **Command Implementations**: Handlers for Application-scoped commands (Quit, Suspend, Navigation, Refresh, Arrange)
 
-The @Terminal.Gui.App.ApplicationImpl class creates and manages the @Terminal.Gui.App.IKeyboard instance, setting its `IApplication` property to `this` to provide the necessary @Terminal.Gui.App.IApplication reference.
+The @Terminal.Gui.App.IApplication implementations create and manage the @Terminal.Gui.App.IKeyboard instance, setting its `IApplication` property to `this` to provide the necessary @Terminal.Gui.App.IApplication reference.
 
 ## Driver