add NetGroupManager

[kouloumos]

This PR

• updates node p2p components with NetGroupManager and related references
  • updates AddrMan description to match that of other components, where a more general introduction is given before going into details.

A questions regarding further contributions: What do you think about the history section as a more explicit way to give context and encourage codebase archaelogy? This guide already references related PRs on sections but I think a more explicit separation will encourage further additions as this guide gets future updates.

[willcl-ark]

This is great, thanks!

I do like the idea of a history section too, and getting more (valuable) contributions is the aim of the game... I'll start trying to incorporate a history sub-section as I remove things to update them myself too.