Rollup merge of #93162 - camsteffen:std-prim-docs, r=Mark-Simulacrum

Std module docs improvements

My primary goal is to create a cleaner separation between primitive types and primitive type helper modules (fixes #92777). I also changed a few header lines in other top-level std modules (seen at https://doc.rust-lang.org/std/) for consistency.

Some conventions used/established:

 * "The \`Box\<T>` type for heap allocation." - if a module mainly provides a single type, name it and summarize its purpose in the module header
 * "Utilities for the _ primitive type." - this wording is used for the header of helper modules
 * Documentation for primitive types themselves are removed from helper modules
 * provided-by-core functionality of primitive types is documented in the primitive type instead of the helper module (such as the "Iteration" section in the slice docs)

I wonder if some content in `std::ptr` should be in `pointer` but I did not address this.

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