Add a conditional banner to the rust docs stating it's out of date

[mdinger]

#22063 suggested adding a banner stating "this version is really old" but modifying rust after the fact wasn't well liked. A better way would be to add a conditional banner which checks rust-lang or something similar to see if something new has been released. It should only appear if it's too old or something similar.

It could even have grades:

• Yellow: soon to be out of date
• Orange: is out of date
• Red: way out of date

Recently it came up on reddit.

[azerupi]

I like the way they do it in the Django docs for example. Bottom right corner, you can choose the version. It's a lot less intrusive than a banner. It's not very obvious, but you could ad an explicit text stating that this version is out of date.

[mdinger]

See also #14466 (comment)

[azerupi]

See also #14466 (comment)

Ow, Sorry for the duplicate :)

And actually I spoke without checking, but Django uses a deprecation banner too for old versions. You can't miss it but it some weird way it still is non-intrusive 😋

Also one thing they have in Django docs, is a box with more information when something has changed recently. Like this:

[steveklabnik]

Triage: this is significantly less of a problem today. MIT's docs went away, and now that things are stable, the most that happens is that someone misses a newer API that could be useful to them. Given that this issue has had no comment in the last year, I'm gonna give it a close.