docs: add specifications for C++ Kernel Runtime, Flattened Device Tree, Physical Memory Manager, SLUB Allocator, and Virtual Memory Manager

- Introduced `cpp_runtime.md` detailing the C++ ABI stubs required for LumeOS kernel, including panic mechanisms and error paths.
- Added `fdt.md` to outline the device tree management for hardware discovery and configuration, including data structures and public APIs.
- Created `pmm.md` to specify the physical memory management system based on the buddy system, including frame allocation and concurrency handling.
- Developed `slab.md` to describe the SLUB allocator for efficient memory allocation, detailing its structure and public APIs.
- Established `vmm.md` to define the virtual memory manager's objectives, dependencies, and core data structures for managing virtual address space.

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