Add pre-merge checklist in core developer guide

[jni]

Our core developer guide has a section on reviewing, but it is missing essential information about merge-time conventions/etiquette. Especially now that @willingc is advocating for getting rid of the "ready to merge" wait period, I think it's important to add to the guide everything that needs to happen before a PR is merged:

• check the type label type (feature, label, performance, etc)
• check the milestone
• check discussion comments are addressed or an issue is made to keep track of remaining to-dos
• make sure the title still reflects the state/purpose of the PR
• make sure the description reflects the current state/purpose of the PR (it's encouraged to leave the original description underneath under the heading "original description", since it provides the context for the subsequent discussion on the PR itself. However, the description becomes the commit message, so it should reflect the status of the PR at the time of merging.)

Some of those checks should ideally be automated, so we could perhaps rename "ready to merge" to "final checks" and keep it only for the automations, not for the wait period.