docs: add new module docs

src/gb.rs

+//! Main GameBoy emulation entrypoint functions and structures.
+
 use std::{
     cell::RefCell,
     collections::VecDeque,

src/mmu.rs

+//! MMU (Memory Management Unit) functions and structures.
+
 use std::{cell::RefCell, rc::Rc};
 
 use crate::{

src/pad.rs

+//! Gamepad related functions and structures.
+
 use crate::warnln;
 
 #[cfg(feature = "wasm")]

src/ppu.rs

+//! PPU (Picture Processing Unit) functions and structures.
+
 use core::fmt;
 use std::{
     borrow::BorrowMut,