Document Migration from `gtk3` to `gtk4` and `webkit2gtk` 4.1 to 6

[tweidinger]

As we are in process of migration to a maintained version of GTK we need to update our documentation to include steps for migration and changes needed for developers coming from V2. This depends on the implementation issues or possible breaking changes in our transition implementation issues. See https://github.com/orgs/tauri-apps/projects/34 for project progress.