docs: add global initialization, linker and entry specifications, and spinlock documentation

- Introduced global initialization sequence documentation outlining the objectives, dependencies, core data structures, public APIs, concurrency contracts, error paths, and state machine for LumeOS.
- Created linker and entry documentation defining early memory layout and assembly startup sequence compatible with multiple bootloaders, including API specifications and error handling.
- Added spinlock documentation detailing the implementation of spinlocks, concurrency synchronization, memory ordering guarantees, and error handling mechanisms.

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