Update lots of doc links to use item paths instead of file paths

[LikeLakers2]

Hi! I noticed that a lot of intra-doc links in rayon-core and rayon uses webpage links (i.e. iter/trait.ParallelExtend.html#tymethod.par_extend) instead of item paths (i.e. iter::ParallelExtend::par_extend()).

This PR converts as many intra-doc links as I could find into item paths. This ensures that if Rustdoc ever changes how its documentation is structured in the file system (unlikely, but not impossible), rayon and rayon-core's docs will continue to function as normal.

This PR also fixes a couple small pieces of documentation which I assume were meant to link to another item.

I made sure to double-check all of my work - and especially made sure the links would work when using cargo +1.63 doc (the msrv of Rayon) - but if you find any documentation links I missed, or any that are now broken, please let me know.

Notes:

• rayon/rayon-core/src/lib.rs

  Line 414 in d179ea9

  /// [`crossbeam::scope`]: https://docs.rs/crossbeam/0.8/crossbeam/fn.scope.html

  - If crossbeam were a dependency of rayon-core, I would be able to change this to an item path. But alas, it is not.

• rayon/rayon-core/src/join/mod.rs

  Line 41 in d179ea9

  /// [the `par_sort` method]: ../rayon/slice/trait.ParallelSliceMut.html#method.par_sort

  - Attempting to change this link to use an item path (i.e. rayon::slice::ParallelSliceMut::par_sort()) results in a broken link warning.

• rayon/src/array.rs

  Line 6 in d179ea9

  //! [arrays]: https://doc.rust-lang.org/std/primitive.array.html

  - I do not know how to link to the primitive using an item path; using std::array links to the std::array module rather than the [T; N] primitive.