Create a consistent set of sections for Edition migrations

[nikomatsakis]

I think every edition migration should have the following sections:

# Summary

*In a nutshell, what changed, and how do I migrate?*

# Motivation and details

*Why was this change made, and give a bit more details about what the change did exactly?*

# Migrating

*What are the precise details for how to migrate, corner cases to be addressed*

[nikomatsakis]

@rustbot assign @rylev

[rylev]

This is done for Rust 2021, but not for the other editions. I'm removing this from the Edition Blockers project.