rustdoc: Way to fold documentation of `Methods from Deref<Target = ...>`

[tesuji]

Take Vec for example, there are fold-able impl blocks, but the inline impl Deref block
cannot be folded. Sometimes I just wanna skim over the doc, and skip uninterested doc block
by folding them.

@rustbot label +A-rustdoc-ui T-rustdoc T-rustdoc-frontend C-enhancement

> rustc --version
Nightly version: 1.81.0-nightly
(2024-07-06 ed7e35f3494045fa1194)