Add a favicon

[csikb]

The project already has an icon, but the website currently lacks a favicon. Adding the favicon will improve the site's visual identity and user experience by ensuring the project icon is visible in browser tabs, bookmarks, and other places where favicons are used.

https://github.com/spring-io/start.spring.io/blob/main/start-client/static/images/icon-48x48.png

The icon is already available, so this should be a matter of generating the correct favicon file(s) and integrating it into the site.

[csikb]

Thanks @oodamien for taking up this task :)