feat: generic components, redirects and frontmatter page titles

[Shewart]

Summary

Closes out the ShellDocs backlog from building the ShellUI docs.

Added

• Generic components in markdown. Generic components register under their bare name. razor:preview, inline tags and <ComponentPreview> close them from a type-parameter attribute, as Razor does (<BarChart TItem="SalesRow" />).

  • Type arguments take C# spellings: aliases, T?, T[], List<T>, simple or full names.
  • Without a type argument, ShellDocs uses object when the constraints allow it, with a warning. Otherwise it renders an inline error.
  • A non-generic component with the same name keeps the tag.
  • <AutoTypeTable> lists type parameters.

• Redirects. URLs that aren't pages are redirected:

  • content folders and version roots go to their first page;
  • unversioned URLs that exist in the latest version go to that page;
  • o.AddRedirect(from, to, permanent) adds segment-aware prefix rules.

  An IStartupFilter adds middleware that answers before routing, so dotted version roots work. It sends 302 for computed redirects and 301 for rules. shelldocs build writes the same redirects as static redirect pages. o.EnableRedirects = false opts out.

• o.RenderPageTitle. Renders the frontmatter title and description as the page header when the body has no # Heading. Off by default.

Changed

• ShellDocs.Components references the ASP.NET Core shared framework, for the middleware, instead of the Components.Web package. It already read content from disk.

Fixed

• shelldocs build prerendered "Page not found" pages when publish copied meta.json but not the .md files. Build now fills in missing content files, and the example csproj is fixed.

Testing

• dotnet build: 0 warnings. dotnet test: 303 passing.
• New test files: GenericComponentTests, RedirectTests, PageTitleTests.
• Example app: every redirect case returns the expected 302 with its target. Pages are untouched and the query string is kept.
• Generic previews render with the right closed types.
• A real shelldocs build wrote 25 pages, none "Page not found", and 14 redirect pages.