Add a migration guide for developers migrating from GTK 3 to GTK 4. There's not much to document for now, but we can add more details on API changes as they are implemented.
Pull request: https://github.com/WebKit/WebKit/pull/5858
Committed 256100@main (99f4041e6cb2): <https://commits.webkit.org/256100@main> Reviewed commits have been landed. Closing PR #5858 and removing active labels.