Add a section on resolving disagreements between core developers

[ncoghlan]

I'm not sure how we'd structure this, but I realised that the principle of "Escalate tracker disagreements to python-ideas and python-dev discussions, and potentially all the way to the PEP process" isn't clearly documented anywhere.

Once a discussion reaches the "This needs a PEP" stage, then there's https://devguide.python.org/langchanges/

I also gave a very brief overview of the concept when describing the extra responsibilities taken on by core developers in https://devguide.python.org/coredev/#what-it-means.

(Prompted by this Twitter thread, where I wanted a link for "Here is how our issue escalation process works", and couldn't find one: https://twitter.com/ncoghlan_dev/status/979228826126360577)