docs: default to IDE Extensions section and add favicon (#12636)

Change the no-match fallback in DocsShell findActiveTab to return the
first nav tab (IDE Extensions) instead of the last (CLI), so the docs
root defaults to IDE Extensions. Add app/favicon.ico so /favicon.ico
requests resolve via App Router metadata instead of falling through the
optional catch-all route.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: sestinj

docs-site/app/favicon.ico

@@ -29,7 +29,7 @@ function findActiveTab(nav: NavTab[], slug: string) {
       }
     }
   }
-  return nav[nav.length - 1];
+  return nav[0];
 }
 
 function slugToLabel(slug: string, titleMap?: Record<string, string>) {