docs: add some migration guide for v6 (#18444)

Author: sapphi-red

docs/guide/migration.md

@@ -10,6 +10,25 @@ Some internal APIs have been removed due to changes in Vite's implementation. If
 
 The experimental Vite Runtime API evolved into the Module Runner API, released in Vite 6 as part of the new experimental [Environment API](/guide/api-environment). Given that the feature was experimental the removal of the previous API introduced in Vite 5.1 isn't a breaking change, but users will need to update their use to the Module Runner equivalent as part of migrating to Vite 6.
 
+## General Changes
+
+### JSON stringify
+
+In Vite 5, when [`json.stringify: true`](/config/shared-options#json-stringify) is set, [`json.namedExports`](/config/shared-options#json-namedexports) was disabled.
+
+From Vite 6, even when `json.stringify: true` is set, `json.namedExports` is not disabled and the value is respected. If you wish to achieve the previous behavior, you can set `json.namedExports: false`.
+
+Vite 6 also introduces a new default value for `json.stringify` which is `'auto'`, which will only stringify large JSON files. To disable this behavior, set `json.stringify: false`.
+
+## Advanced
+
+There are other breaking changes which only affect few users.
+
+- [[#18209] refactor!: bump minimal terser version to 5.16.0](https://github.com/vitejs/vite/pull/18209)
+  - Minimal supported terser version for [`build.minify: 'terser'`](/config/build-options#build-minify) was bumped to 5.16.0 from 5.4.0.
+- [[#18243] chore(deps)!: migrate `fast-glob` to `tinyglobby`](https://github.com/vitejs/vite/pull/18243)
+  - Range braces (`{01..03}` ⇒ `['01', '02', '03']`) and incremental braces (`{2..8..2}` ⇒ `['2', '4', '6', '8']`) are no longer supported in globs.
+
 ## Migration from v4
 
 Check the [Migration from v4 Guide](https://v5.vite.dev/guide/migration.html) in the Vite v5 docs first to see the needed changes to port your app to Vite 5, and then proceed with the changes on this page.