docs: add info about changes in localized fields to v2 -> v3 migration guide (#11244)

### What?
Information that locale fields in database are changing to a simpler
data structure in v3.

### Why?
Simple data migration is not enough to get it working, I had to spend
quite some time to figure out migration files and still it required some
manual input. Maybe others will find it useful when starting a v3
migration.

### How?
I had to do some custom migration scripts on my own to get v3 to work
with existing pages data.

---------

Co-authored-by: Sasha <64744993+r1tsuu@users.noreply.github.com>

Author: inuue

docs/migration-guide/overview.mdx

@@ -414,6 +414,15 @@ For more details, see the [Documentation](https://payloadcms.com/docs/getting-st
     ```
 1. The `./src/public` directory is now located directly at root level `./public` [see Next.js docs for details](https://nextjs.org/docs/pages/building-your-application/optimizing/static-assets)
 
+1.  Payload now automatically removes `localized: true` property from sub-fields if a parent is localized, as it's redunant and unnecessary. If you have some existing data in this structure and you want to disable that behavior, you need to enable `allowLocalizedWithinLocalized` flag in your payload.config [read more in documentation](https://payloadcms.com/docs/configuration/overview#compatibility-flags), or create a migration script that aligns your data.
+Mongodb example for a link in a page layout.
+
+    ```diff
+    - layout.columns.en.link.en.type.en
+    + layout.columns.en.link.type
+    ```
+
+
 ## Custom Components
 
 1. All Payload React components have been moved from the `payload` package to `@payloadcms/ui`. If you were previously importing components into your app from the `payload` package, for example to create Custom Components, you will need to change your import paths: