Added documentation about potential XSS in router.push (vercel#71645)

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### What?
Currently, the `router.push` method does not sanitize URL arguments,
which can cause cross-site scripting (XSS) bugs in next.js sites through
the running of untrusted code in a JavaScript URL. This was reported as
vercel#50093 and was closed, with an explanation that it is the developer's
responsibility to sanitize `router.push` input. This PR is an addition
to the next.js docs to document that issue and let developers know that
they cannot send untrusted or unsanitized URLs into `router.push`.

### Why?
Cross-site scripting bugs can be quite high severity, as they often
allow attackers to steal credentials and data. Furthermore, the API most
similar to `router.push` in the web API, `history.pushState`, does not
accept JavaScript URLs, so developers might reasonably not know that
they need to sanitize `router.push` input. Searching through public
github repos finds more than 1,000 projects that may be vulnerable to
this issue, so I believe it's a pretty widespread misunderstanding of
the API.

docs/02-app/02-api-reference/04-functions/use-router.mdx

@@ -50,6 +50,7 @@ export default function Page() {
 
 > **Good to know**:
 >
+> - You must not send untrusted or unsanitized URLs to `router.push` or `router.replace`, as this can open your site to cross-site scripting (XSS) vulnerabilities. For example, `javascript:` URLs sent to `router.push` or `router.replace` will be executed in the context of your page.
 > - The `<Link>` component automatically prefetch routes as they become visible in the viewport.
 > - `refresh()` could re-produce the same result if fetch requests are cached. Other Dynamic APIs like `cookies` and `headers` could also change the response.