feat(em): add banner for custom install (#4738)

* feat(em): add banner for custom install

* Update pages/elastic-metal/how-to/configure-disk-partitions.mdx

Co-authored-by: Jessica <[email protected]>

---------

Co-authored-by: Jessica <[email protected]>

Author: bene2k1

pages/elastic-metal/how-to/configure-disk-partitions.mdx

@@ -28,6 +28,13 @@ However, if you have specific requirements, you can define a custom partition la
 - An [SSH key](/organizations-and-projects/how-to/create-ssh-key/)
 - An [Elastic Metal server](/elastic-metal/quickstart/#how-to-create-an-elastic-metal-server)
 
+<Message type="important">
+  Custom partitioning of an Elastic Metal server is only possible if:
+  - The server configuration includes faster installation and custom partitioning.
+  - It is done during the first installation or a reinstallation.
+  - Once installed, partitioning cannot be changed without a reinstallation.
+</Message>
+
 ## Simplified partitioning (visual interface)
 
 During [server installation](/elastic-metal/how-to/install-server/), navigate to step 5 of the creation wizard and click **Custom Configuration** > **Simplified Partitioning**.