Add Announcement Bar for Express 5.1.0 Release

[ShubhamOulkar]

With the release of Express 5.1.0, we should add an announcement bar above the navigation menu to inform users about the latest version. The announcement message should be:

Now Express 5.1.0 is available on npm.

Need approval from @expressjs/express-tc, @expressjs/docs-wg

[bjohansebas]

Yep, we just need to enable the announcement again, it's in the Jekyll configuration.

I can do it later if no one else has opened the PR when I get back home, or tomorrow. Thanks for the reminder, I had forgotten we had that.