Update Documentation on the IDT

[gmarino2048]

The explanation for Chapter 3.1 is very sparse and could use improvement. Update the IDT section with all of the new information about the structure of 64-bit interrupts, and how interrupts can be used to transition from one context to another. You should also add a section about why the Double Fault IRQ seems to be sent every time the IDT is initialized on QEMU.