_This is a follow-up for #3718 and #3922._ Write content for `docs/site/migration/extensions.md`, explain how can users migrate their custom components to LB4. ## Acceptance criteria - [ ] How to rewrite components from LB3 to LB4 style - [ ] How to migrate applications using such components - [ ] What's our advice for apps using official loopback components like `loopback-component-storage`?