[rustdoc] Some items don't have documentation rendered

[GuillaumeGomez]

This comes from #112308: some items like non-inlined re-exports or pub extern crate don't have any documentation rendered although they appear in documentation. The reason is that they don't have their own page. Something that was suggested was to group items that "couldn't be implementated using impl block" into their own page (like constants, statics, non-inlined re-exports, etc). So all constants would into a same constants.html page. But that would be for later since it'd change URLs, requiring an RFC.

The motivation behind this is to allow all items into the documentation to have the same behaviour: on the module page, short documentation. On their "own" (grouped) page, full documentation.

What do you think @rust-lang/rustdoc?