Add comprehensive documentation

[LordHerdier]

@amorrisCS Could you check over the interrupt and timer docs to make sure everything looks okay?

@BryanSimp Keyboard implementation still a WIP, so does the keyboard doc look right?

@carlitodavis Mind looking over the memory docs?

[amorrisCS]

idt.md, pic.md, and pit.md all look great!

[carlitodavis]

Memory doc looks really strong overall — the phase breakdown and explanation of mmap parsing, bump allocation, and the bitmap allocator are all clear and match the system well.

A couple small things to note if SCRUM 7 merges.

• The PMM initialization section describes a “mark-all-used then free usable regions” approach, but the current allocator initializes the bitmap as free and explicitly reserves kernel/heap and module regions afterward. Might be worth clarifying which model we’re following to avoid confusion.

• The reservation step could be a bit more explicit — especially that we now reserve from managed_base up to memory_base_address() and also walk mb->mods_addr to reserve multiboot modules. Those details are pretty important for correctness and were the source of earlier bugs.

• Minor naming mismatch: doc refers to pmm_init() while code uses page_alloc_init() — not a blocker, but worth aligning for clarity.