Writing the migration guide for Kivy 2.3.0 apps to 3.0.0

[misl6]

Writing a migration guide for porting Kivy 2.3.0 apps to 3.0.0 could be an enormous amount of work to do if done on a single shot.

Instead, when making changes via PRs that will impact 2.x.x features, is the author's responsibility (with maintainer help) to keep a list of the things that have been removed/moved/changed and to write migration suggestions and guide.

Every time a new PR is ready to be merged:

• Check if any breaking change has been made
• If a breaking change has been made, add a new point to MIGRATION-2.3.x_to_3.0.0.md, explain what has been changed, and suggest a migration path
• Discuss it with the PR reviewer, and improve it
• Ping a maintainer, and get it approved

At the end of the 3.0.0 milestone, and before the release:

• Review MIGRATION-2.3.x_to_3.0.0.md, and add relevant warnings on docs.
• Include MIGRATION-2.3.x_to_3.0.0.md in docs.