Issue #3590402: Views pages don't respond to Accept: text/markdown and ?_format=markdown triggers.

Closes #3590402

Entity routes have a dedicated route with _format: markdown on the original path, which is what makes Accept: text/markdown, Content-Type: text/markdown and ?_format=markdown work. MarkdownifyViewsRoutes only creates the /markdownify/{path} route, so those three triggers fail on Views pages. The fix adds a second route on the original view path with _format: markdown pointing to the same controller (MarkdownifyViewPageController). This makes Views pages consistent with entity routes across all supported triggers.