docs(dev): Add backport considerations to development process

[joshtrichards]

☑️ Resolves

This is an attempt to describe a few of the considerations about "when a backport should be done".

To my knowledge there is no formal policy about backports other than "critical bug fixes". The only true "no-brainer" backports are security matters.

This is partially about documenting things as they are today, but it's unavoidably also a policy question. (It's also probable I've slipped a few of my own biases into this).

Related: nextcloud/backportbot#385

🖼️ Screenshots