Update deepdives to capture new Scrolling API

[Copilot]

Proposed Changes/Todos

Updates scrolling.md and layout.md to document the new Scrolling API that replaced ScrollBar.AutoShow with ScrollBarVisibilityMode enum and ViewportSettingsFlags.

• Document ScrollBarVisibilityMode enum (Manual, Auto, Always, None)
• Document ViewportSettingsFlags.HasVerticalScrollBar and HasHorizontalScrollBar
• Expand ViewportSettings section with all flag categories
• Replace all AutoShow references with new API patterns
• Update code examples in View.md, migratingfromv1.md, newinv2.md

New API Pattern:

// Old (deprecated)
view.VerticalScrollBar.AutoShow = true;

// New (recommended)
view.ViewportSettings |= ViewportSettingsFlags.HasVerticalScrollBar;

// Alternative (manual control)
view.VerticalScrollBar.VisibilityMode = ScrollBarVisibilityMode.Always;

Updated Documentation:

• scrolling.md - Added ScrollBarVisibilityMode section, expanded ViewportSettings with ScrollBar flags
• layout.md - Added ScrollBar flags to ViewportSettings section
• View.md, migratingfromv1.md, newinv2.md - Updated all code examples

Pull Request checklist:

• I've named my PR in the form of "Fixes #issue. Terse description."
• My code follows the style guidelines of Terminal.Gui - if you use Visual Studio, hit CTRL-K-D to automatically reformat your files before committing.
• My code follows the Terminal.Gui library design guidelines
• I ran dotnet test before commit
• I have made corresponding changes to the API documentation (using /// style comments)
• My changes generate no new warnings
• I have checked my code and corrected any poor grammar or misspellings
• I conducted basic QA to assure all features are working

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Deepdives do not capture new Scrolling API</issue_title>
<issue_description>These deep dives need updating based on the merged #4714

• scrolling.md
• layout.md

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Deepdives do not capture new Scrolling API #4746

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