fix(docs): update tanstack start routing setup

[mehdibha]

The current Tanstack Start routing setup doesn't handle external links properly. When passing a string href (e.g., <MenuItem href="https://github.com">), the router's navigate and buildLocation methods fail with:

String.prototype.search called on null or undefined

Added type guards to handle string hrefs separately:

<RouterProvider
  navigate={(href, opts) => {
    if (typeof href === "string") return;
    router.navigate({ ...href, ...opts });
  }}
  useHref={(href) => {
    if (typeof href === "string") return href;
    return router.buildLocation(href).href;
  }}
>