add migration guide for graphiql v5 (#3988)

Author: dimaMachina

docs/migration/graphiql-5.0.0.md

@@ -0,0 +1,15 @@
+# Upgrading `graphiql` from `4.x` to `5.0.0-rc`
+
+---
+
+## `graphiql`
+
+- Migration from Codemirror to
+  [Monaco Editor](https://github.com/microsoft/monaco-editor)
+  - Replacing `codemirror-graphql` with
+    [`monaco-graphql`](https://github.com/graphql/graphiql/tree/main/packages/monaco-graphql)
+  - Clicking on a reference in the query editor now works by holding `Cmd` on macOS or `Ctrl` on Windows/Linux
+- Support for comments in **Variables** and **Headers** editors
+- Added new examples: [**GraphiQL x Vite**](https://github.com/graphql/graphiql/tree/graphiql-5/examples/graphiql-vite) and [**GraphiQL x
+  Next.js**](https://github.com/graphql/graphiql/tree/graphiql-5/examples/graphiql-nextjs)
+- Removed examples: **GraphiQL x Parcel** and **GraphiQL x Create React App**