Begin restructuring (#25)

* Restructure ToC and add placeholders for future pages

* Add missing section placeholders

Author: SonoSooS

content/SUMMARY.md

@@ -1,22 +1,89 @@
 # GBA Doc
 
 * [Intro](intro.md)
-* [CPU](cpu.md)
-* [Memory](memory.md)
-* [Graphics Hardware Overview](graphics.md)
-* [Backgrounds](backgrounds.md)
-* [OAM (Sprites)](sprites.md)
-* [Windowing](windowing.md)
-* [Hardware Interrupts](interrupts.md)
-* [BIOS (Software Interrupts)](bios.md)
-* [Memory-Mapped Hardware Registers](registers.md)
-* [Audio](audio/introduction.md)
+* [System Overview]()
+* [Memory Layout](memory.md)
+* [Built-in Peripherals]()
+  * [IO Register Summary](registers.md)
+  * [Graphics](graphics.md)
+    * [Backgrounds](backgrounds.md)
+    * [OAM (Sprites)](sprites.md)
+    * [Windowing](windowing.md)
+    * [Mosaic]()
+    * [Blending]()
+    * [Stereoscopy]()
+  * [Audio](audio/introduction.md)
     * [Direct Sound](audio/directsound.md)
-    * [Sound Channel 1](audio/sound1.md)
-    * [Sound Channel 2](audio/sound2.md)
-    * [Sound Channel 3](audio/sound3.md)
-    * [Sound Channel 4](audio/sound4.md)
+    * [DMG Sound]()
+      * [Sound Channel 1](audio/sound1.md)
+      * [Sound Channel 2](audio/sound2.md)
+      * [Sound Channel 3](audio/sound3.md)
+      * [Sound Channel 4](audio/sound4.md)
     * [Sound Registers](audio/registers.md)
-* [Fixed-Point Math for Newbies](fixed-point-math.md)
-* [Bootleg Carts](bootleg-carts/introduction.md)
+  * [DMA]()
+  * [Timers]()
+  * [External IO]()
+    * [Keypad Buttons]()
+    * [Serial Port]()
+      * [Normal / SPI]()
+      * [Multiplay]()
+      * [JOYBUS]()
+      * [UART]()
+      * [GPIO]()
+  * [Interrupt Controller]()
+  * [System Control]()
+    * [Cartridge Slot]()
+    * [Power-Saving Features]()
+    * [Test Mode]()
+    * [Game Boy Color mode]()
+* [Programmers' Reference]()
+  * [CPU](cpu.md)
+  * [BIOS]()
+    * [Built-in Functions (SWIs)](bios.md)
+      * [System Functions]()
+      * [Math Functions]()
+      * [Memory Functions]()
+      * [Decompression Functions]()
+      * [Sound Functions]()
+      * [MusicPlayer Functions]()
+      * [Multiboot Function]()
+    * [BIOS Memory]()
+    * [Startup Sequence]()
+    * [Multiboot]()
+    * [MusicPlayer]()
+  * [Interrupt Handling](interrupts.md)
+  * [Cartridges]()
+    * [Cartridge Header]()
+    * [Save Types]()
+      * [SRAM]()
+      * [EEPROM]()
+      * [Flash]()
+    * [Cartridge Hardware]()
+      * [GPIO Cartridge]()
+        * [Cartridge RTC]()
+        * [Cartridge Light Sensor]()
+        * [Cartridge Rumble]()
+        * [Cartridge Gyroscope]()
+      * [Cartridge Accelerometer]()
+      * [e-Reader]()
+  * [Accessories]()
+    * [Game Boy Player]()
+    * [Wireless Adapter]()
+* [Hardware Specifications]()
+  * [Pinouts]()
+    * [Cartridge]()
+    * [LCD]()
+    * [EXT Ports]()
+    * [SoC]()
+  * [Signals and Buses]()
+    * [Cartridge Protocol]()
+    * [LCD Driving Waveforms]()
+    * [EXRT Port Signals]()
+      * [SIO SPI]()
+      * [SIO Multiplay]()
+      * [SIO JOYBUS]()
+      * [SIO UART]()
+* [Guides]()
+  * [Fixed-Point Math for Newbies](fixed-point-math.md)
+  * [Bootleg Carts](bootleg-carts/introduction.md)
 * [Acknowledgements](ack.md)