Add trait object safety information on trait documentation page

[GuillaumeGomez]

Fixes #85138.

It looks like this:

So now, what remains to be done is debating over the appearance of the information.

r? @jsha

[jsha]

Thanks for working on this! Two issues:

• This puts the "this trait is object safe" text in a very high-priority place, right at the very top of each trait page. But it's relatively low-priority information: most uses of traits are not using trait objects. Also, putting it at the top of so many trait pages will make beginners feel like they need to learn about trait objects and object safety right away, even though they are an intermediate-skill part of the language.

• We put English text in the UI, which I think is fine for now - i18n would be a big project. But there are relatively few places we inject English text into the documentation itself. The main place I can think of is #[non_exhaustive], and I've seen some interest in changing that to just be a link to the doc page that explains the non_exhaustive attribute. An issue with injecting English text in the documentation is that it may be jarring when the rest of the documentation is non-English.

Let's brainstorm some more UI over in the issue thread.

[JohnCSimon]

triage: Looks like this is waiting on author
@rustbot label: +S-waiting-on-author -S-waiting-on-review

[GuillaumeGomez]

This is definitely not. We didn't take time to talk about it yet. It's blocked instead.

[bors]

☔ The latest upstream changes (presumably #102061) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnCSimon]

@GuillaumeGomez
Ping from triage: I'm closing this due to inactivity, Please reopen when you are ready to continue with this.
Note: if you do please open the PR BEFORE you push to it, else you won't be able to reopen - this is a quirk of github.
Thanks for your contribution.

@rustbot label: +S-inactive