Add migration guide from @team-plain/typescript-sdk

Step-by-step guide covering package rename, import changes,
enum-to-string-literal migration, error handling updates,
webhook and UI component package splits, and pagination changes.

Author: mattvagni

.changeset/proud-wings-poke.md

@@ -0,0 +1,5 @@
+---
+"@team-plain/sdk": minor
+---
+
+Update GraphQL schema