Add P2P/networking docs

[lrettig]

https://docs.google.com/document/d/1fyASTE83bcSyLecyKOFS4CNeMhmjtwWxu-_ymLI2S2M/edit

Should replace https://github.com/spacemeshos/go-spacemesh/wiki/p2p2 and https://github.com/spacemeshos/go-spacemesh/wiki/P2P-Architecture

Yosher shared these newer notes, check whether they contain anything relevant for this doc:

• https://docs.google.com/document/d/1cj_9sI5hQjYSf9aezuHxwxxJkzXa5Wm8JCcxsJWO_CE/edit?usp=sharing
• https://hackmd.io/16TewoG5QBilLaXzwONk8w?both

[lrettig]

Note to self: ask @YaronWittenstein for review