Document ipdevpoll design choices for developers

[lunkwill42]

Describe the bug

Just about the only reference documentation of ipdevpoll that exists as of today is this page, which is geared towards end users: https://github.com/Uninett/nav/blob/master/doc/reference/ipdevpoll.rst

However, there is no reference documentation for developers that want to get involved in hacking on ipdevpoll. The closest thing is this howto guide: https://github.com/Uninett/nav/blob/master/doc/hacking/adding-environment-probe-support.rst

Expected behavior

There should be some reference documentation on ipdevpoll that explains its architecture and design choices, in order to aid new contributors in finding their way to develop new functionality in ipdevpoll.

Keywords to keep in mind:

• Twisted, event loops and asynchronous code considerations
• Choice of SNMP library for asynchronous code
• Jobs and plugins
• Collection phase vs. storage phase
• Shadow classes vs. Django ORM models