Auto merge of #83068 - mockersf:method-trait-foreign-impl, r=GuillaumeGomez

Auto merge of #83068 - mockersf:method-trait-foreign-impl, r=GuillaumeGomez

rustdoc: links from items in a trait impl are inconsistent

Depending on where the struct implementing a trait is coming from, or the current page, the items in a trait impl are not linking to the same thing:

|item| trait page, implementors| trait page, implementations on Foreign Types|struct page, trait implementations|
|-|-|-|-|
|function|             link to current impl|link to first impl in the list|link to trait def
|default function |    not present         |not present                   |link to trait def
|default function with custom impl|link to current impl|link to trait def             |link to trait def
|constant|             link to current impl|link to trait def             |link to trait def
|associated type|      link to current impl|link to trait def             |link to trait def
||*missing link to trait def*|*function link wrong + missing link to current impl*|*missing link to current impl*|

<details>
  <summary>rust code with those cases</summary>

```rust
pub trait MyTrait {
    type Assoc;
    const VALUE: u32;
    fn trait_function(&self);
    fn defaulted(&self) {}
    fn defaulted_override(&self) {}
}

impl MyTrait for String {
    /// will link to trait def
    type Assoc = ();
    /// will link to trait def
    const VALUE: u32 = 5;
    /// will link to first foreign implementor
    fn trait_function(&self) {}
    /// will link to trait def
    fn defaulted_override(&self) {}
}

impl MyTrait for Vec<u8> {
    /// will link to trait def
    type Assoc = ();
    /// will link to trait def
    const VALUE: u32 = 5;
    /// will link to first foreign implementor
    fn trait_function(&self) {}
    /// will link to trait def
    fn defaulted_override(&self) {}
}

impl MyTrait for MyStruct {
    /// in trait page, will link to current impl
    ///
    /// in struct page, will link to trait def
    type Assoc = bool;
    /// in trait page, will link to current impl
    ///
    /// in struct page, will link to trait def
    const VALUE: u32 = 20;
    /// in trait page, will link to current impl
    ///
    /// in struct page, will link to trait def
    fn trait_function(&self) {}
    /// in trait page, will link to current impl
    ///
    /// in struct page, will link to trait def
    fn defaulted_override(&self) {}
}

pub struct MyStruct;
```
</details>

In this PR, I fixed all links to target the trait definition, and added an anchor-link to the current implementation appearing on mouse hover.