Annotate server migration

Annotate server migration with a note in regard to migration between different architectures and the need to dump/restore the database.

docs/maintenance-migrating.md

@@ -1,3 +1,8 @@
+> **Note**: This migration guide is applicable if you migrate from server to another that are of the same architecture, i.e. e.g. the servers both are amd64. 
+> 
+> In case that you migrate between two different architectures (e.g. amd64 --> arm64), simply copying the complete folder is not possible as it would move the raw PostgreSQL data between different architectures. In this specific case, you would need to dump the database on your current server and import it properly on the new server.
+> 
+
 # Migrating to new server
 
 1. Prepare by lowering DNS TTL for your domains (`matrix.DOMAIN`, etc.), so that DNS record changes (step 4 below) would happen faster, leading to less downtime