Rollup merge of #75685 - nixphix:docs/unix-ext, r=jyn514

Rollup merge of #75685 - nixphix:docs/unix-ext, r=jyn514

Switch to intra-doc links in /src/sys/unix/ext/*.rs

Partial fix for #75080

@rustbot modify labels: T-doc, A-intra-doc-links, T-rustdoc

r? @jyn514

These two links are not resolving to either `crate::fs::File...` or `fs::File...`
```
# unix/ext/fs.rs
   27:    /// [`File::read`]: ../../../../std/fs/struct.File.html#method.read

  130:   /// [`File::write`]: ../../../../std/fs/struct.File.html#method.write
```