HACKING.rst: introduce .net -> Networking refactor section

[OddBloke]

As I was looking at the initial implementation for this refactor, I realised that we didn't actually have a concrete description of the proposed refactor (and process). To save people from having to Just Know (TM), this introduces a section to HACKING.md named Ongoing Refactors as well as its first entry, for this refactor.

I do not intend this to be merged before I have an initial implementation (hence "Draft"), but figured that I may as well push this up for review while I'm working on that.

[OddBloke]

For ease of review, the rendered version of HACKING.md can be seen here: https://people.canonical.com/~dwatkins/pr384/topics/hacking.html#ongoing-refactors

[raharper]

Thanks Dan, this is really well put together.

[igalic]

I'm no seeing any mention of EphemeralIPv4Network or EphemeralDHCPv4, but i suppose that's another big refractor that can happen in another iteration.
baby steps.

[OddBloke]

Given that we've agreed on this as a basis going forward, I actually think it makes sense to land this in-tree so that we can more easily discuss required amendments to it as I do the initial implementation. I've marked it as ready for review, and should be able to land it (with Ryan's review) once CI passes.