Add kmem_cache explanation to memory allocation section

[visitorckw]

Introduces the kmem_cache for frequent, fixed-size allocations.

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Summary by cubic

Adds a runnable kmem_cache example and expands the memory allocation chapter to show when and how to use slab caches for frequent, fixed-size objects. Includes examples/kmem_cache.c, a new kmem_cache target in examples/Makefile, and lkmpg.tex guidance with a sample link plus usage of kmem_cache_create(), kmem_cache_alloc(), kmem_cache_free(), and kmem_cache_destroy().

^{Written for commit 8350828. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 3 files

_{Re-trigger cubic}

[jserv]

Thank @visitorckw for contributing!