Add a banner telling the user they're not looking at the latest version

[jaskij]

Quite often, I'd look up a crate in a search engine, and get linked an outdated version for no good reason. From there, getting to the latest version requires:

• realizing it happened
• clicking "Versions"
• waiting the short time for a page load
• choosing the latest version

The part about realizing I'm looking at the wrong thing is what takes the longest, and usually only happens when I already spent some time looking at whatever else I came looking for,

I'd love if there was a small banner telling me that I'm in fact not looking at the latest version. It does not have to be big or very noticeable - something small and unobtrusive like docs.rs does would be amazing.

[LawnGnome]

Closing in favour of #10710, which is the same thing but with a bit more detail.