Add (very basic) v5 migration guide (#6589)

Author: martinbonnin

docs/source/migration/5.0.mdx

@@ -0,0 +1,18 @@
+---
+title: Migrating to Apollo Kotlin 5
+subtitle: Step-by-step guide on migrating from Apollo Kotlin 4
+description: Learn how to migrate from version 4 with key changes, new features, and tools designed for improved stability and maintainability. Follow step-by-step guidance and make a seamless transition
+---
+
+⚠️ This is a work in progress ⚠️
+
+Behaviour changes:
+
+* [ ] https://github.com/apollographql/apollo-kotlin/pull/6455
+* [ ] https://github.com/apollographql/apollo-kotlin/pull/6485
+
+Removed APIs
+
+* [ ] https://github.com/apollographql/apollo-kotlin/pull/6494
+* [ ] https://github.com/apollographql/apollo-kotlin/pull/6492
+* [ ] #6496