rustdoc: Quick way to collapse all impl blocks

[crumblingstatue]

The primary motivation of #130612 was to provide an easy way to overview the different kinds of impls on a type.
#132155 fulfills the layout aspect of the motivation, but there is no way (to my knowledge) to quickly collapse all impl blocks with one click.
The user has to manually click collapse on each one.
There should be a quick and convenient way to collapse (and expand) all the impl blocks to make it convenient to overview them.

The question is what would be the best way.
Here are some possibilities:

1. Change the behavior of the summary button to also collapse impl blocks. Would this be desirable or annoying for users?
2. Add another button. Would it clutter the UI too much?
3. Add an alternate way to use the summary button. For example a shift+click could also collapse/expand impl blocks.
   A hover tooltip on the button could inform the user about this alternate usage.