doc: explain how to use functional indexes in migrations (#4149)

examples/functionalidx/atlas.hcl

@@ -0,0 +1,15 @@
+data "composite_schema" "app" {
+  # Load the ent schema first with all tables.
+  schema "public" {
+    url = "ent://ent/schema"
+  }
+  # Then, load the functional indexes.
+  schema "public" {
+    url = "file://schema.sql"
+  }
+}
+
+env "local" {
+  src = data.composite_schema.app.url
+  dev = "docker://postgres/15/dev?search_path=public"
+}