docs: Add more documents (but they're unfinished)

docs/instructions.md

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+# Fox32 instructions
+
+This file describes every Fox32 instruction in detail. For a general
+description of the Fox32 CPU, see cpu.md.
+
+## NOP: no operation
+## ADD: add
+## MUL: multiply (unsigned)
+## AND: bitwise AND
+## SLA: shift left
+## SRA: shift right arithmetic (with sign extension)
+## BSE: bit set
+## CMP: compare
+## JMP: absolute jump
+## RJMP: relative jump
+## PUSH: push value to stack
+## IN: get input from I/O bus
+## ISE: set interrupt enable flag
+## MSE: set MMU enable flag
+## HALT: halt CPU
+## INC: increment (add 1)
+## OR: bitwise OR
+## IMUL: multiply (signed)
+## SRL: shift right logical (with zero extension)
+## BCL: bit clear
+## MOV: move value
+## CALL: absolute call
+## RCALL: relative call
+## POP: pop value from stack
+## OUT: output on I/O bus
+## ICL: clear interrupt enable flag
+## MCL: clear MMU enable flag
+## BRK: debug breakpoint
+## SUB: subtract
+## DIV: divide (unsigned)
+## XOR: bitwise XOR
+## ROL: rotate left
+## ROR: rotate right
+## BTS: test if bit set
+## MOVZ: move value and clear upper bits in target register
+## LOOP: absolute loop
+## RLOOP: relative loop
+## RET: return from function
+## INT: raise interrupt
+## TLB: flush TLB and set page directory pointer
+## DEC: decrement (subtract 1)
+## REM: calculate remainder of division (unsigned)
+## NOT: bitwise NOT
+## IDIV: divide (signed)
+## IREM: remainder (signed)
+## RTA: calculate address relative to instruction pointer
+## RETI: return from interrupt
+## FLP: flush page from TLB

docs/io_bus.md

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+# I/O bus
+
+|  start     |  end       | description
+|------------|------------|---------------------------------------
+| 0x00000000 | 0x00000000 | debug serial port
+| 0x80000000 | 0x8000031f | display overlay
+| 0x80000400 | 0x80000401 | mouse
+| 0x80000500 | 0x80000500 | keyboard
+| 0x80000600 | 0x80000600 | audio
+| 0x80000700 | 0x80000706 | RTC
+| 0x80001000 | 0x80005003 | disk
+| 0x80010000 | 0x80010000 | power off
+
+
+TODO: details

docs/memory.md

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+# Memory
+
+The memory in a Fox32 system is laid out as follows:
+
+|  start     |  end       |  size   | description
+|------------|------------|---------|---------------------------------------
+| 0x00000000 | 0x04000000 |  64 MiB | RAM
+| 0xf0000000 | 0xf0080000 | 512 KiB | ROM