Rollup merge of #130798 - lukas-code:doc-stab, r=notriddle

rustdoc: inherit parent's stability where applicable

It is currently not possible for a re-export to have a different stability (rust-lang/rust#30827). Therefore the standard library uses a hack when moving items like `std::error::Error` or `std::net::IpAddr` into `core` by marking the containing module (`core::error` / `core::net`) as unstable or stable in a later version than the items the module contains.

Previously, rustdoc would always show the *stability as declared* for an item rather than the *stability as publicly reachable* (i.e. the features required to actually access the item), which could be confusing when viewing the docs. This PR changes it so that we show the stability of the first unstable parent or the most recently stabilized parent instead, to hopefully make things less confusing.

fixes rust-lang/rust#130765

screenshots:
![error in std](https://github.com/user-attachments/assets/2ab9bdb9-ed81-4e45-a832-ac7d3ba1be3f) ![error in core](https://github.com/user-attachments/assets/46f46182-5642-4ac5-b92e-0b99a8e2496d)

Author: tgross35