refactor: moved the migration guide to the version after the next major update

Author: mfranzke

docs/migration/v5.x.x-to-v6.0.0.md

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+## Migration (3.x.x) ➡ 4.0.0
+
+## Refactoring of Header and Navigation
+
+We encountered some problems with the current `DBHeader` as well with the `DBNavigation`.
+By splitting the `DBHeader` into 2 components (`DBControlPanelDesktop` & `DBControlPanelMobile`),
+we are able to control our navigation based on the device size.
+Additionally, we split the `DBNavigationItem` as well to provide a new `DBNavigationItemGroup` instead of
+having a Slot with `subNavigation`.
+
+For a detailed migration based on framework checkout those:
+
+- [Angular](300-400/shell/Angular.md)
+- [React](300-400/shell/React.md)
+- [Vue](300-400/shell/Vue.md)
+
+### Changes in DBPage ➡️ DBShell
+
+- **Mandatory Usage**: The `DBShell` component is now mandatory and replaces the optional `DBPage`.
+- **Removed Properties**: The following properties from `DBPage` are no longer supported in `DBShell`:
+    - `documentOverflow`
+    - `variant`
+    - `footer`
+- **Slot Changes**:
+    - The `header` slot has been split into two new slots:
+        - `controlPanelDesktop`
+        - `controlPanelMobile`
+
+### Changes in DBHeader ➡️ DBControlPanelDesktop/DBControlPanelMobile
+
+- **Component Split**: The `DBHeader` has been split into two components:
+    - `DBControlPanelDesktop`
+    - `DBControlPanelMobile`
+- **Slot Wrapping**: The following slots now require additional wrapping components:
+    - `brand` ➡️ `DBControlPanelBrand`
+    - `metaNavigation` ➡️ `DBControlPanelMetaNavigation`
+    - `primaryActions` ➡️ `DBControlPanelPrimaryActions`
+    - `secondaryActions` ➡️ `DBControlPanelSecondaryActions`
+
+### Changes in Navigation ➡️ NavigationItemGroup
+
+- **Slot Removal**: The `subNavigation` slot in `NavigationItem` has been removed.
+- **New Component**: Use the new `NavigationItemGroup` component to group navigation items instead of relying on the `subNavigation` slot.
+
+#### Example Migration
+
+Before:
+
+```html
+<NavigationItem><a>Item</a></NavigationItem>
+<NavigationItem>
+	Group
+	<template slot:sub-navigation>
+		<NavigationItem><a>Sub Item 1</a></NavigationItem>
+		<NavigationItem><a>Sub Item 2</a></NavigationItem>
+	</template>
+</NavigationItem>
+```
+
+After:
+
+```html
+<NavigationItem><a>Item</NavigationItem>
+<NavigationItemGroup text="Group">
+	<NavigationItem><a>Sub Item 1</a></NavigationItem>
+	<NavigationItem><a>Sub Item 2</a></NavigationItem>
+</NavigationItemGroup>
+```
+
+## Automate migration via CLI
+
+We provide a CLI tool to auto migrate your source code. Use this command in your repository:
+
+```shell
+npx @db-ux/core-migration --type=v200_v300 --src=./src
+```