Add nanoc 4 upgrade guide

A work in progress, especially considering nanoc 4 is not finished yet :)

I’d still like to merge this as soon as possible, so that there’s a definitive resource for tracking nanoc 4 changes.

To do:

* [x] Mark page as draft
* [x] Link to page from Development section
* [x] Write “Compact rules” section
* [x] Write “Extracted plugins” section