Add a section on resolving disagreements between core developers

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)

まず、devguide.python.org/coredev/#what-it-means と devguide.python.org/langchanges/ にある既存のセクションを読んでください。tracker 上の意見の不一致をエスカレーションする新しいセクションをどこに追加するのが適切かを特定し、python-ideas、python-dev、PEP プロセスを経る手順を、リンクと明確な停止点を含めて文書化してください。