rustdoc: migrate item_keyword to an askama template

[nicklimmm]

Part of #108868

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @GuillaumeGomez (or someone else) soon.

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[GuillaumeGomez]

I'm not sure it's worth it here. It adds a lot of code and the template only calls the document function...

[nicklimmm]

I'm not sure it's worth it here. It adds a lot of code and the template only calls the document function...

I do feel the same while implementing this. Do feel free to close this PR then.

[GuillaumeGomez]

Don't hesitate to take a look at the other items on the list.