Convert Custom View Builder to modal dialog and improve UX

[Malcolmnixon]

This pull request updates the documentation to reflect the migration of the custom view builder from a docked panel to a new modal dialog (ViewBuilderDialog) in the SysML2Workbench AppShellSubsystem. It introduces a new design document for ViewBuilderDialog, updates integration and UI composition details, and removes references to the now-deleted docked custom view builder panel throughout the docs. The changes clarify the dialog's role, data flow, error handling, and integration with the shell, ensuring consistency across the design documentation.

Major documentation updates:

Introduction of ViewBuilderDialog:

• Added a new design document view-builder-dialog.md detailing the purpose, data model, methods, error handling, and dependencies of ViewBuilderDialog, which replaces the docked custom view builder panel.
• Registered the new review unit for ViewBuilderDialog in .reviewmark.yaml, including all relevant source, view, and test files.

UI and Dock Layout Changes:

• Updated the dock layout documentation to remove references to the docked custom view builder panel, clarifying that only three tool panels remain docked and that ViewBuilderDialog is a modal dialog, not part of the persistent Dock layout. [1] [2] [3] [4] [5] [6]
• Updated UI composition and MVVM documentation to reference ViewBuilderDialogViewModel instead of the deleted CustomViewBuilderToolViewModel. [1] [2]

Integration and Behavior Updates:

• Documented how the dialog is opened from the View menu, how it manages its own state, and its interaction with the shell for preview and commit actions. [1] [2]
• Clarified the single code path for SysML snippet generation and clipboard actions, now routed through the diagram tab context menu and no longer through the deleted panel. [1] [2]

Consistency and Error Handling:

• Described how the dialog handles empty workspace states and error reporting, ensuring the user experience is consistent and recoverable. [1] [2]

These changes ensure the documentation accurately reflects the current architecture and user flow, focusing on the new ViewBuilderDialog and its integration into the AppShellSubsystem.