Add and maintain a migration guide

[yrodiere]

So that people can know what broke, like here.

For inspiration, see:

• https://github.com/hibernate/hibernate-orm/blob/main/migration-guide.adoc
• https://github.com/hibernate/hibernate-search/blob/main/documentation/src/main/asciidoc/migration/index.adoc