fix: sidebar index pages, phone-width previews and tables

[Shewart]

Summary

Fixes found while moving the ShellUI docs to ShellDocs 0.1.10 and ShellUI 0.3.2.

Fixed

• Sidebar listed a folder's index.md twice: as the section label and again as a page with the same title ("Components" / "Components", "Form" / "Form Components"). The label now links to the index page and is highlighted when you're on it, and the page isn't repeated. Collapsible sections keep a separate chevron toggle.
• Collapsed sidebar sections had no aria-expanded. Blazor drops a false boolean attribute, so the toggle now writes "true" / "false".
• Wide markdown tables widened the page on phones. Rendered tables sit in a .shelldocs-table box that scrolls horizontally.

Added

• scroll preview layout. razor:preview scroll, or Layout="scroll" on <DemoPreview> / <ComponentPreview>, lets an example wider than the frame (pagination, toolbars, OTP inputs) scroll inside it instead of spilling past it. It's opt-in, because a scroll box clips popovers.

Changed

• On phones (≤480px) the preview frame's side padding drops from 1.5rem to 0.75rem.
• Centring uses safe center, so an example that's still too wide starts at the left edge instead of losing both sides.

Testing

• dotnet build: 0 warnings. dotnet test: 306 passing, including the new SidebarIndexPageTests, a table-wrapping test and scroll-layout cases.
• Checked against the ShellUI docs site at 358px:
  • all 70 component pages, with no demo spilling past its frame and no page scrolling sideways;
  • the sidebar shows each index page once;
  • the card footer demo fits.