doc: add memory section

[rakita]

Add docs about memory and opcodes that change it.

[github-actions]

Valgrind Results:

==3985== Cachegrind, a cache and branch-prediction profiler
==3985== Copyright (C) 2002-2017, and GNU GPL'd, by Nicholas Nethercote et al.
==3985== Using Valgrind-3.18.1 and LibVEX; rerun with -h for copyright info
==3985== Command: target/release/snailtracer
==3985== 
--3985-- warning: L3 cache found, using its data for the LL simulation.
Snailtracer Host+Interpreter benchmark (2.1s) ...            not enough samples
==3985== 
==3985== I   refs:      463,579,345
==3985== I1  misses:          7,745
==3985== LLi misses:          2,987
==3985== I1  miss rate:        0.00%
==3985== LLi miss rate:        0.00%
==3985== 
==3985== D   refs:      237,187,747  (160,464,875 rd   + 76,722,872 wr)
==3985== D1  misses:        324,312  (    184,150 rd   +    140,162 wr)
==3985== LLd misses:        137,567  (      4,065 rd   +    133,502 wr)
==3985== D1  miss rate:         0.1% (        0.1%     +        0.2%  )
==3985== LLd miss rate:         0.1% (        0.0%     +        0.2%  )
==3985== 
==3985== LL refs:           332,057  (    191,895 rd   +    140,162 wr)
==3985== LL misses:         140,554  (      7,052 rd   +    133,502 wr)
==3985== LL miss rate:          0.0% (        0.0%     +        0.2%  )