Move to intra-doc links for library/core/src/alloc/{layout, global, mod}.rs
Helps with #75080. The files already contained intra-doc links, so there are only minor changes.
@rustbot modify labels: T-doc, A-intra-doc-links, T-rustdoc
Known issues:
* [`handle_alloc_error`]: Link from `core` to `alloc` could not be resolved.
* [`slice`]: slice is a primitive type, but could not be resolved; had to use [`crate::slice`] instead.
527a685e40