[rustdoc] Inline impl docstring headers when possible

[crumblingstatue]

#141663 Added a way to collapse all impl blocks, which is one step closer to a great reading experience when using separate impls for splitting up your API documentation.

However, the impl blocks are currently spaced too far apart for a comfortable overview.

Here is an example viewing the egui::Ui documentation:

A possible improvement would be to determine (possibly using some kind of heuristic) if it's possible to inline the first paragraph or header of the doc string, and if so, inline it.

Here is a mockup made in an image editor:

This would make it much easier to overview impls.

The heuristic can be conservative. For starters, maybe it could just trigger for the simple case of impl Item, without any generics or bounds, to make it simple to implement.