Auto merge of #106561 - GuillaumeGomez:warning-block, r=rustdoc

Add warning block support in rustdoc

Fixes rust-lang/rust#79710.

You can test it [here](https://rustdoc.crud.net/imperio/warning-block/foo/struct.Foo.html). It currently looks like this:

![image](https://user-images.githubusercontent.com/3050060/211413494-e1cf04e4-c081-4a9d-97db-27329405cfa7.png)

So a few things to note:

 * Since it's a new add and it's changing the UI, we'll need to go through an FCP.
 * Does the UI looks good?
 * Is the way picked to add a warning block ok for everyone? The discussion on the issue seemed to be in favour of this solution but it doesn't hurt to double-check.

cc `@rust-lang/rustdoc`

Author: bors