Add warning block support in rustdoc Fixes https://github.com/rust-lang/rust/issues/79710. You can test it [here](https://rustdoc.crud.net/imperio/warning-block/foo/struct.Foo.html). It currently looks like this:  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`